qdoc3: qdoc user manual update
qdoc3: qdoc user manual update

Added descriptions for the \ditamap, \topicref, and
\mapref commands.

Task-number:  QTBUG-24425
Change-Id: Ibbb1945c501be88ad7e715c67a6ce0a921bfb29b
Reviewed-by: Casper van Donderen <808524cef5c654648bea2a2b954f3b3445ed083c@nokia.com>

kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc