Add \brief descriptions to QtConcurrent topics

Fixes: QTBUG-104213
Pick-to: 6.4 6.3 6.2
Change-Id: Ib787546ef74133aca4b963a4341283147d2e8fb2
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/concurrent/qtconcurrentfilter.cpp

@@ -4,6 +4,7 @@
 /*!
     \page qtconcurrentfilter.html
     \title Concurrent Filter and Filter-Reduce
+    \brief Selecting values from a sequence and combining them, all in parallel.
     \ingroup thread
 
     The QtConcurrent::filter(), QtConcurrent::filtered() and

src/concurrent/qtconcurrentmap.cpp

@@ -108,6 +108,7 @@
 /*!
     \page qtconcurrentmap.html
     \title Concurrent Map and Map-Reduce
+    \brief Transforming values from a sequence and combining them, all in parallel.
     \ingroup thread
 
     The QtConcurrent::map(), QtConcurrent::mapped() and

src/concurrent/qtconcurrentrun.cpp

@@ -4,6 +4,7 @@
 /*!
     \page qtconcurrentrun.html
     \title Concurrent Run
+    \brief A simple way to run a task in a separate thread.
     \ingroup thread
 
     The QtConcurrent::run() function runs a function in a separate thread.

src/concurrent/qtconcurrenttask.qdoc

@@ -4,6 +4,7 @@
 /*!
     \page qtconcurrenttask.html
     \title Concurrent Task
+    \brief A configurable way to run a task in a separate thread.
     \ingroup thread
 
     QtConcurrent::task provides an alternative interface for running a