media: doc/media api: Try to make enum usage clearer

This clarifies which side of the calls is responsible for doing what to
which parts of the struct. This also explicitly states that repeating
values are disallowed. It also expands the terse description of the access
algorithm into more prose-like, active voice description, which trades
conciseness for ease of comprehension.

Added: mbus codes must not repeat
Added: no holes in the enumeration
Added: enumerations per what?
Added: who fills in what in calls
Changed: "zero" -> "0"
Changed: "given" -> "specified"

Still unclear how it works so didn't describe: "which". What is a "try
format" vs "active format"?

[Sakari Ailus: Rewrap lines, fix build issue]

Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@puri.sm>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>