drm/amd/display: Add Freesync video documentation

Recently, we added support for an experimental feature named Freesync
video; for more details on that, refer to:

commit 6f59f229 ("drm/amd/display: Skip modeset for front porch change")
commit d10cd527 ("drm/amd/display: Add freesync video modes based on preferred modes")
commit 0eb1af2e ("drm/amd/display: Add module parameter for freesync video mode")

Nevertheless, we did not document it in detail in our driver. This
commit introduces a kernel-doc and expands the module parameter
description.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>