Moc: document new command line argument '-M'

Moc has been updated to be able to append additional
meta data tags to plugins. This change will
document that feature.

Change-Id: I7cb669eb2e72ee846b857112305dc2b588fd7990
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>