drm/amd/display: Expand kernel doc for DC

[ Upstream commit 1682bd1a ]

This commit adds extra documentation for elements related to FAMs.

Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Reviewed-by: Aurabindo Pillai <Aurabindo.Pillai@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Stable-dep-of: 67e38874 ("drm/amd/display: Increase num voltage states to 40")
Signed-off-by: Sasha Levin <sashal@kernel.org>