QDebug: Add a doc section about formatting options

Change-Id: I43cb965f9f45e8a50c767c7e9035811c90e6aab3
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Fix warnings about integer conversion in qloggingregistry.cpp.

io\qloggingregistry.cpp(134) : warning C4267: 'argument' : conversion from 'size_t' to 'int', possible loss of data
io\qloggingregistry.cpp(138) : warning C4267: 'argument' : conversion from 'size_t' to 'int', possible loss of data
io\qloggingregistry.cpp(142) : warning C4267: 'argument' : conversion from 'size_t' to 'int', possible loss of data

Change-Id: Ic9787aa8acb5cb4440c62bbb143f7c2b7fdad385
Reviewed-by: Kai Koehne <kai.koehne@digia.com>