drm/amd/display: Start documentation of DC

[Why]
There are a lot of unintuitive parts of the dm-dc interface.
It would help us if these were documented to provide
a common understanding of what they are supposed to do

[How]
Most of this documentation is stubs, to be filled out more
thoroughly by the experts

Not every dm-accessible function and struct is mentioned.
Simple functions like getters, setters, retain, release,
create, destroy can be left unadorned.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>