Update QMutexLocker documentation

And add an entry about the API changes to qt6-changed.qdoc.

Pick-to: 6.0
Task-number: QTBUG-89429
Change-Id: I146574acd3e4aa5ed6f1629fc68888f009472f19
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/doc/snippets/code/src_corelib_thread_qmutex.cpp

@@ -184,7 +184,7 @@ int complexFunction(int flag)
 class SignalWaiter
 {
 private:
-    QMutexLocker locker;
+    QMutexLocker<QMutex> locker;
 
 public:
     SignalWaiter(QMutex *mutex)

src/corelib/doc/src/qt6-changes.qdoc

@@ -392,6 +392,15 @@
         }
     \endcode
 
+    \section1 QMutex and Related Classes
+
+    In Qt 6, QRecursiveMutex does not inherit from QMutex anymore. This change was
+    done to improve the performance of both QMutex and QRecursiveMutex.
+
+    Due to those changes, the QMutex::RecursionMode enum has been removed, and
+    QMutexLocker is now a templated class that can operate on both QMutex and
+    QRecursiveMutex.
+
     \section1 QFuture and Related Classes
 
     \section2 QFuture

src/corelib/thread/qmutex.cpp

@@ -415,7 +415,7 @@ void QRecursiveMutex::unlock() noexcept
 
 
 /*!
-    \class QMutexLocker
+    \class template <typename Mutex> QMutexLocker
     \inmodule QtCore
     \brief The QMutexLocker class is a convenience class that simplifies
     locking and unlocking mutexes.