Doc: Added brief statement to group threads

Change-Id: I81b54b9707a9ef52920924e66a07681a1f829259
Task-number: QTBUG-42682
Reviewed-by: Martin Smith <martin.smith@digia.com>

doc/src/frameworks-technologies/threads.qdoc

@@ -27,6 +27,7 @@
 
 /*!
     \group thread
+    \brief How to develop multithreaded applications.
     \title Threading Classes
 
     These \l{Qt Core} classes provide threading support to applications.