V4L/DVB (5336): Cx23416 doc updates + rename CX2341X_ENC_UNKNOWN

The documentation of Several miscellaneous commands was updated.
As a result of which the CX2341X_ENC_UNKNOWN command was renamed to
CX2341X_ENC_SET_VERT_CROP_LINE.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>