Doc: Added brief statement to group definition

Groups: richtext and sharing.
Task-number: QTBUG-42682
Change-Id: I46bd7e5bba0f665519ee4f3c033b971f0836e314
Reviewed-by: Martin Smith <martin.smith@digia.com>

src/corelib/doc/src/implicit-sharing.qdoc

@@ -30,6 +30,7 @@
 
 /*!
     \group shared
+    \brief How to maximize resource usage by implicit data sharing.
     \title Implicitly Shared Classes
 
     These \l{Qt Core} classes provides a safe and efficient way of sharing and

src/corelib/doc/src/statemachine.qdoc

@@ -27,6 +27,7 @@
 
 /*!
     \group statemachine
+    \brief How to create and execute state graphs.
     \title State Machine Classes
 
     These \l{Qt Core} classes are part of the \l{The State Machine Framework}{

src/gui/doc/src/richtext.qdoc

@@ -27,6 +27,7 @@
 
 /*!
     \group richtext-processing
+    \brief How to use Rich Text Processing APIs.
     \title Rich Text Processing APIs
 */