media: media-entity: Clarify media_entity_cleanup() usage

Being able to call cleanup functions on objects that haven't been
initialized but whose memory has been zeroed simplifies error handling.
The media_entity_cleanup() function documentation doesn't tell whether
this is allowed or not, and inspection of its implementation doesn't
provide any clue as the function is currently empty. Update the
documentation to explicitly allow this usage pattern.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>