drm/doc: Document adjusted/request modes a bit better

Laurent started a massive discussion on IRC about this. Let's try to
document common usage a bit better.

v2: Cross-links+typos.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170515091136.26307-1-daniel.vetter@ffwll.ch

include/drm/drm_modeset_helper_vtables.h

@@ -147,7 +147,8 @@ struct drm_crtc_helper_funcs {
 	 * encoders need to be fed with. Note that this is the inverse semantics
 	 * of the meaning for the &drm_encoder and &drm_bridge_funcs.mode_fixup
 	 * vfunc. If the CRTC cannot support the requested conversion from mode
-	 * to adjusted_mode it should reject the modeset.
+	 * to adjusted_mode it should reject the modeset. See also
+	 * &drm_crtc_state.adjusted_mode for more details.
 	 *
 	 * This function is used by both legacy CRTC helpers and atomic helpers.
 	 * With atomic helpers it is optional.
@@ -528,7 +529,8 @@ struct drm_encoder_helper_funcs {
 	 * mode is the display mode that should be fed to the next element in
 	 * the display chain, either the final &drm_connector or a &drm_bridge.
 	 * The parameter adjusted_mode is the input mode the encoder requires. It
-	 * can be modified by this callback and does not need to match mode.
+	 * can be modified by this callback and does not need to match mode. See
+	 * also &drm_crtc_state.adjusted_mode for more details.
 	 *
 	 * This function is used by both legacy CRTC helpers and atomic helpers.
 	 * This hook is optional.