[media] DocBook: make the G/S/TRY_FMT specification more strict

- S/TRY_FMT should always succeed, unless an invalid type field is passed in.
- TRY_FMT should give the same result as S_FMT, all other things being equal.
- ENUMFMT may return different formats for different inputs or outputs.
This was decided during the 2012 Media Workshop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>