[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.