Add \brief descriptions to CMake overviews

Add brief descriptions of the pages that are shown in
  https://doc.qt.io/qt-6/overviews.html

Change-Id: I177e4ba82cd7b8e264122375bf9a595509918fdd
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>
(cherry picked from commit 2e241d153738c9881c4e57a6b8f095cd1c514992)

src/corelib/doc/src/cmake/cmake-commands.qdoc

@@ -4,6 +4,7 @@
 /*!
 \group cmake-commands-qtcore
 \title CMake Commands in Qt6 Core
+\brief Lists CMake commands defined in Qt6::Core.
 
 The following CMake commands are defined when Qt6::Core is loaded, for instance
 with

src/corelib/doc/src/cmake/cmake-configure-variables.qdoc

@@ -10,6 +10,7 @@
 /*!
 \group cmake-variables-qtcore
 \title CMake Variables in Qt6 Core
+\brief Lists CMake variables defined in Qt6::Core.
 
 The following CMake variables are defined when Qt6::Core is loaded, for instance
 with

src/corelib/doc/src/cmake/cmake-properties.qdoc

@@ -4,6 +4,7 @@
 /*!
 \group cmake-target-properties-qtcore
 \title CMake Target Properties in Qt6 Core
+\brief Lists CMake target properties known to Qt6::Core.
 
 \l{CMake Commands in Qt6 Core}{CMake Commands} know about the following CMake
 target properties:
@@ -395,6 +396,7 @@ resource prefix.
 /*!
 \group cmake-source-file-properties-qtcore
 \title CMake Source File Properties in Qt6 Core
+\brief Lists CMake file properties used in Qt6::Core.
 
 \l{CMake Commands in Qt6 Core}{CMake Commands} know about the following CMake
 source file properties:

src/dbus/doc/src/qtdbus-cmake.qdoc

@@ -4,6 +4,7 @@
 /*!
 \group cmake-commands-qtdbus
 \title CMake Commands in Qt6 DBus
+\brief Lists CMake commands defined in Qt6::DBus.
 
 The following CMake commands are defined when Qt6::DBus is loaded, for instance
 with
@@ -246,6 +247,7 @@ argument.
 /*!
 \group cmake-source-file-properties-qtdbus
 \title CMake Source File Properties in Qt6 DBus
+\brief Lists CMake file properties used in Qt6::DBus.
 
 \l{CMake Commands in Qt6 DBus}{CMake Commands} know about the following CMake
 source file properties: