qdoc: Moved qdoc comments from qmacstyle_mac.mm to qstyles.qdoc 

The .mm file is not read by qdoc for packages other than for the MAC.
This problem is overcome by moving the qdoc comments from the .mm file
to a .qdoc file in doc/src, because all these files are read by qdoc 
for each of the packages.

#Task-number: 252566