Add note about first-class callables to callable type and add link to the closure documentation page (php#4792)

Author: shaedrich

language/types/callable.xml

@@ -18,9 +18,10 @@
   <title>Passing</title>
 
   <para>
-   A PHP function is passed by its name as a <type>string</type>. Any built-in
-   or user-defined function can be used, except language constructs such as:
-   <function>array</function>, <function>echo</function>,
+   A PHP function is passed by either its name as a <type>string</type> or by
+   a <link linkend="functions.first_class_callable_syntax">first-class callable</link>.
+   Any built-in or user-defined function can be used, except language constructs
+   such as: <function>array</function>, <function>echo</function>,
    <function>empty</function>, <function>eval</function>,
    <function>exit</function>, <function>isset</function>,
    <function>list</function>, <function>print</function> or
@@ -125,7 +126,7 @@ call_user_func($c, 'PHP!');
   <para>
    <example>
     <title>
-     Callback example using a Closure
+     Callback example using a <classname>Closure</classname>
     </title>
    <programlisting role="php">
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