drm/modes: Fix description of DRM_MODE_TYPE_USERDEF

These days DRM_MODE_TYPE_USERDEF is used to flag modes defined via the
kernel command line. Update the docs to reflect that fact.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114183258.16976-6-ville.syrjala@linux.intel.com
Acked-by: Alex Deucher <alexander.deucher@amd.com>