Mention that QtMessageHandler needs to be reentrant,
as well as other caveats. Mention QLoggingCategory,
so people do know that they don't have to necessarily
implement their own handler to filter messages (and that
not all messages reach the handler). Also mention
qFormatLogMessage().
Finally, give a more useful example for a custom
message handler that logs to a file. Note that the example
leaks a file handle at exit, but that is arguably not that
bad.
Pick-to: 6.5 6.6
Change-Id: I5be44167b266c9bbdbb0e94806bb024c9b352a32
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
cb841b449a