drm/doc: Discourage usage of MODESET_CTL ioctl

Leftover from the old days of ums and should be used any longer. Since

commit 29935554b384b1b3a7377d6f0b03b21d18a61683
Author: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Date:   Wed May 30 00:58:09 2012 +0200

    drm: Disallow DRM_IOCTL_MODESET_CTL for KMS drivers

it is a complete no-Op for kms drivers.

v2: Fix up mangled sentence spotted by Michel.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Michel Dänzer <michel@daenzer.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -2861,12 +2861,12 @@ int num_ioctls;</synopsis>
             <term>DRM_IOCTL_MODESET_CTL</term>
             <listitem>
               <para>
-                This should be called by application level drivers before and
-                after mode setting, since on many devices the vertical blank
-                counter is reset at that time.  Internally, the DRM snapshots
-                the last vblank count when the ioctl is called with the
-                _DRM_PRE_MODESET command, so that the counter won't go backwards
-                (which is dealt with when _DRM_POST_MODESET is used).
+		This was only used for user-mode-settind drivers around
+		modesetting changes to allow the kernel to update the vblank
+		interrupt after mode setting, since on many devices the vertical
+		blank counter is reset to 0 at some point during modeset. Modern
+		drivers should not call this any more since with kernel mode
+		setting it is a no-op.
               </para>
             </listitem>
           </varlistentry>