Add documentation for async() function and Fiber-based await()

Author: clue

README.md

@@ -16,6 +16,7 @@ an event loop, it can be used with this library.
 **Table of Contents**
 
 * [Usage](#usage)
+    * [async()](#async)
     * [await()](#await)
     * [coroutine()](#coroutine)
     * [parallel()](#parallel)
@@ -53,6 +54,145 @@ use React\Async;
 Async\await(…);
 ```
 
+### async()
+
+The `async(callable $function): callable` function can be used to
+return an async function for a function that uses [`await()`](#await) internally.
+
+This function is specifically designed to complement the [`await()` function](#await).
+The [`await()` function](#await) can be considered *blocking* from the
+perspective of the calling code. You can avoid this blocking behavior by
+wrapping it in an `async()` function call. Everything inside this function
+will still be blocked, but everything outside this function can be executed
+asynchronously without blocking:
+
+```php
+Loop::addTimer(0.5, React\Async\async(function() {
+    echo 'a';
+    React\async\await(React\Promise\Timer\sleep(1.0));
+    echo 'c';
+}));
+
+Loop::addTimer(1.0, fn() => echo 'b'));
+
+// prints "a" at t=0.5s
+// prints "b" at t=1.0s
+// prints "c" at t=1.5s
+```
+
+See also the [`await()` function](#await) for more details.
+
+Note that this function only works in tandem with the [`await()` function](#await).
+In particular, this function does not "magically" make any blocking function
+non-blocking:
+
+```php
+Loop::addTimer(0.5, React\Async\async(function() {
+    echo 'a';
+    sleep(1); // broken: using PHP's blocking sleep() for demonstration purposes
+    echo 'c';
+}));
+
+Loop::addTimer(1.0, fn() => echo 'b'));
+
+// prints "a" at t=0.5s
+// prints "c" at t=1.5s: Correct timing, but wrong order
+// prints "b" at t=1.5s: Triggered too late because it was blocked
+```
+
+As an alternative, you should always make sure to use this function in tandem
+with the [`await()` function](#await) and an async API returning a promise.
+
+The `async()` function is specifically designed for cases where it is used
+as a callback (such as an event loop timer, event listener, or promise
+callback). For this reason, it returns a new function wrapping the given
+`$function` instead of directly invoking it and returning its value.
+
+```php
+use function React\Async\async;
+
+Loop::addTimer(1.0, async(function () { … }));
+$connection->on('close', async(function () { … }));
+$stream->on('data', async(function ($data) { … }));
+$promise->then(async(function (int $result) { … }));
+```
+
+You can invoke this wrapping function to invoke the given `$function` with
+any arguments given as-is. The function will always return a Promise which
+will be fulfilled with whatever your `$function` returns. Likewise, it will
+return a promise that will be rejected if you throw an `Exception` or
+`Throwable` from your `$function`. This allows you to easily create
+Promise-based functions:
+
+```php
+$promise = React\Async\async(function (): int {
+    $browser = new React\Http\Browser();
+    $urls = [
+        'https://example.com/alice',
+        'https://example.com/bob'
+    ];
+
+    $bytes = 0;
+    foreach ($urls as $url) {
+        $response = React\Async\await($browser->get($url));
+        assert($response instanceof Psr\Http\Message\ResponseInterface);
+        $bytes += $response->getBody()->getSize();
+    }
+    return $bytes;
+})();
+
+$promise->then(function (int $bytes) {
+    echo 'Total size: ' . $bytes . PHP_EOL;
+}, function (Exception $e) {
+    echo 'Error: ' . $e->getMessage() . PHP_EOL;
+});
+```
+
+The previous example uses [`await()`](#await) inside a loop to highlight how
+this vastly simplifies consuming asynchronous operations. At the same time,
+this naive example does not leverage concurrent execution, as it will
+essentially "await" between each operation. In order to take advantage of
+concurrent execution within the given `$function`, you can "await" multiple
+promises by using a single [`await()`](#await) together with Promise-based
+primitives like this:
+
+```php
+$promise = React\Async\async(function (): int {
+    $browser = new React\Http\Browser();
+    $urls = [
+        'https://example.com/alice',
+        'https://example.com/bob'
+    ];
+
+    $promises = [];
+    foreach ($urls as $url) {
+        $promises[] = $browser->get($url);
+    }
+
+    try {
+        $responses = React\Async\await(React\Promise\all($promises));
+    } catch (Exception $e) {
+        foreach ($promises as $promise) {
+            $promise->cancel();
+        }
+        throw $e;
+    }
+
+    $bytes = 0;
+    foreach ($responses as $response) {
+        assert($response instanceof Psr\Http\Message\ResponseInterface);
+        $bytes += $response->getBody()->getSize();
+    }
+    return $bytes;
+})();
+
+$promise->then(function (int $bytes) {
+    echo 'Total size: ' . $bytes . PHP_EOL;
+}, function (Exception $e) {
+    echo 'Error: ' . $e->getMessage() . PHP_EOL;
+});
+```
+
 ### await()
 
 The `await(PromiseInterface $promise): mixed` function can be used to
@@ -63,8 +203,27 @@ $result = React\Async\await($promise);
 ```
 
 This function will only return after the given `$promise` has settled, i.e.
-either fulfilled or rejected. While the promise is pending, this function will 
-suspend the fiber it's called from until the promise is settled.
+either fulfilled or rejected. While the promise is pending, this function
+can be considered *blocking* from the perspective of the calling code.
+You can avoid this blocking behavior by wrapping it in an [`async()` function](#async)
+call. Everything inside this function will still be blocked, but everything
+outside this function can be executed asynchronously without blocking:
+
+```php
+Loop::addTimer(0.5, React\Async\async(function() {
+    echo 'a';
+    React\async\await(React\Promise\Timer\sleep(1.0));
+    echo 'c';
+}));
+
+Loop::addTimer(1.0, fn() => echo 'b'));
+
+// prints "a" at t=0.5s
+// prints "b" at t=1.0s
+// prints "c" at t=1.5s
+```
+
+See also the [`async()` function](#async) for more details.
 
 Once the promise is fulfilled, this function will return whatever the promise
 resolved to.
@@ -125,10 +284,11 @@ when the promise is fulfilled. The `yield` statement returns whatever the
 promise is fulfilled with. If the promise is rejected, it will throw an
 `Exception` or `Throwable`.
 
-The `coroutine()` function will always return a Proimise which will be
+The `coroutine()` function will always return a Promise which will be
 fulfilled with whatever your `$function` returns. Likewise, it will return
 a promise that will be rejected if you throw an `Exception` or `Throwable`
-from your `$function`. This allows you easily create Promise-based functions:
+from your `$function`. This allows you to easily create Promise-based
+functions:
 
 ```php
 $promise = React\Async\coroutine(function () {