BlackBerry: Improve platform specific documentation

Updated BlackBerry specific documentation around QSettings to make the
differences more obvious for developers.

Back-ported from qtbase: 563342d7ef7e490239cba6d335849ebb91983b9a

Change-Id: I603053cecff97f3222257cbba8d72d2ec2666d6f
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>