drm/atomic: Improve docs for drm_atomic_state->allow_modeset

Motivated by vmwgfx digging around in core uapi bits it shouldn't dig
around in.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004202446.22905-7-daniel.vetter@ffwll.ch