[media] DocBook: Fix remaining issues with VB2 core documentation

Right now, "private:" tag should be lower-case, otherwise the
scripts/kernel-doc won't do the right thing.

Also, no fields after "private:" should be documented. As we don't
want to strip the documentation, let's untag. This way, it will
be seen only at the file, and not at the DocBooks.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>