drm/amd/display: Document set RECOUT operation

During the investigation on how to add visual confirmation on top of the
planes used by DCN, it becomes evident that the lack of information in
the code makes this work unnecessarily complicated. This commit
introduces a set of documentation related to the RECOUT operation in
order to make it easy for developers to navigate this set of functions.

Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>