clarify paragraph in "On using std::sync::Mutex and tokio::sync::Mutex" (#836)

Regarding to how the synchronous and asynchronous `Mutex` behave with the executor.

Author: conectado

content/tokio/tutorial/shared-state.md

@@ -98,9 +98,10 @@ from within async code. An async mutex is a mutex that is locked across calls
 to `.await`.
 
 A synchronous mutex will block the current thread when waiting to acquire the
-lock. This, in turn, will block other tasks from processing. However, switching
-to `tokio::sync::Mutex` usually does not help as the asynchronous mutex uses a
-synchronous mutex internally.
+lock. This, in turn, will block other tasks from processing. Switching
+to `tokio::sync::Mutex` will cause the task to yield control back to the
+executor, but this will usually not help with performance as the asynchronous
+mutex uses a synchronous mutex internally.
 
 As a rule of thumb, using a synchronous mutex from within asynchronous code is
 fine as long as contention remains low and the lock is not held across calls to