drm/doc: vblank cleanup

Unify and review everything, plus make sure it's all correct markup.
Drop the kernel-doc for internal functions. Also rework the overview
section, it's become rather outdated.

Unfortuantely the kernel-doc in drm_driver isn't rendered yet, but
that will change as soon as drm_driver is kernel-docified properly.

Also document properly that drm_vblank_cleanup is optional, the core
calls this already.

v2: Make it clear that cleanup happens in drm_dev_fini for drivers
with their own ->release callback (Thierry).

Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-11-daniel.vetter@ffwll.ch
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 629a5fe..3a911a6 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -685,6 +685,9 @@ struct drm_crtc_funcs {
 	 * drm_crtc_vblank_off() and drm_crtc_vblank_on() when disabling or
 	 * enabling a CRTC.
 	 *
+	 * See also &drm_device.vblank_disable_immediate and
+	 * &drm_device.max_vblank_count.
+	 *
 	 * Returns:
 	 *
 	 * Raw vblank counter value.