drm/doc: add WARNING for drm_device::struct_mutex

The mutex should be used, only by legacy drivers. Add a big warning to
deter people from using it.

Suggested-by: Daniel Vetter <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Emil Velikov <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_device.h

···
 	 * @struct_mutex:
 	 *
 	 * Lock for others (not &drm_minor.master and &drm_file.is_master)
+	 *
+	 * WARNING:
+	 * Only drivers annotated with DRIVER_LEGACY should be using this.
 	 */
 	struct mutex struct_mutex;