drm/doc: switch drm_connector_state to inline comments

For consistency. Also spelled out the docs for ->best_encoder a bit
more while at it.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-4-daniel.vetter@ffwll.ch