[media] Docbook: Fix comments at v4l2-mem2mem.h

Warning(.//include/media/v4l2-mem2mem.h:50): No description found for parameter 'lock'
Warning(.//include/media/v4l2-mem2mem.h:50): No description found for parameter 'unlock'
Warning(.//include/media/v4l2-mem2mem.h:167): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:177): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:188): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:197): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:206): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:215): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:226): No description found for parameter 'm2m_ctx'
Warning(.//include/media/v4l2-mem2mem.h:235): No description found for parameter 'm2m_ctx'

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>