drm/amdgpu/disply: fix documentation warnings in display manager

Add documentation for crc window.

Fixes: c920888c ("drm/amd/display: Expose new CRC window property")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Wayne Lin <Wayne.Lin@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>