[media] DocBook: update descriptions for the media controller entities

Cleanup the media controller entities description:
- remove MEDIA_ENT_T_DEVNODE and MEDIA_ENT_T_V4L2_SUBDEV entity
  types, as they don't mean anything;
- add MEDIA_ENT_T_UNKNOWN with a proper description;
- remove ALSA and FB entity types. Those should not be used, as
  the types are deprecated. We'll soon be adidng ALSA, but with
  a different entity namespace;
- improve the description of some entities.

Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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