drm/amd/display: add doc entries for MPC blending configuration

drm/amd/display: add doc entries for MPC blending configuration

Describe structs and enums used to set blend mode properties to MPC
blocks. Some pieces of information are already available as code
comments, and were just formatted. Others were collected and summarised
from discussions on AMD issue tracker[1][2].

[1] https://gitlab.freedesktop.org/drm/amd/-/issues/1734
[2] https://gitlab.freedesktop.org/drm/amd/-/issues/1769

v2:
- fix typos (Tales)
- add MPCC to MPC entry in the glossary

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>