media: vb2: add cross references at memops and v4l2 kernel-doc markups

Add cross-references where needed and add periods at the end of
each kernel-doc paragraph, in order to make it coherent with other
VB2 descriptions.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>