[docs] QScopeGuard: mention dismiss()ing in \class

The class docs should include a discussion of salient API, not leave
that only to \fn's.

Amends 9229452e574e603aa86fd5adb28da33cf5879db9.

Pick-to: 6.10 6.9 6.8 6.5
Change-Id: I53144c7e56eca21ceeda0c1bca0028e987da9e18
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qscopeguard.qdoc

@@ -18,6 +18,9 @@ QT_BEGIN_NAMESPACE
     your cleanup code is executed, whether the function is exited normally,
     exited early by a return statement, or exited by an exception.
 
+    A scope guard can be disabled using dismiss(), in which case the function
+    is not run at all.
+
     \note Exceptions are not supported. The callable shouldn't throw when
           executed, copied or moved.