Doc: Remove convoluted details of uncommon QMutex use case

Change-Id: If80068f44fef7ab0692b9dad99aa7e1a30cb72a6
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/corelib/doc/src/threads-basics.qdoc

@@ -266,15 +266,6 @@
     used to make a method thread safe. Most Qt methods aren't thread safe
     because there is always a performance penalty when using mutexes.
 
-    It isn't always possible to lock and unlock a mutex in a method. Sometimes
-    the need to lock spans several calls. For example, modifying a container
-    with an iterator requires a sequence of several calls which should not be
-    interrupted by other threads. In such a scenario, locking can be achieved
-    with a mutex that is kept outside of the object to be manipulated. With an
-    external mutex, the duration of locking can be adjusted to the needs of the
-    operation. One disadvantage is that external mutexes aid locking, but do
-    not enforce it because users of the object may forget to use it.
-
     \section2 Dealing with Asynchronous Execution
 
     One way to obtain a worker thread's result is by waiting for the thread