drm/amd/display: kernel-doc: document force_timing_sync

As warned when running "make htmldocs":

	./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:345: warning: Function parameter or member 'force_timing_sync' not described in 'amdgpu_display_manager'

This new struct member was not documented at kernel-doc markup.

Fixes: 3d4e52d0 ("drm/amd/display: Add debugfs for forcing stream timing sync")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -149,6 +149,8 @@ struct amdgpu_dm_backlight_caps {
  * @cached_state: Caches device atomic state for suspend/resume
  * @cached_dc_state: Cached state of content streams
  * @compressor: Frame buffer compression buffer. See &struct dm_comressor_info
+ * @force_timing_sync: set via debugfs. When set, indicates that all connected
+ *		       displays will be forced to synchronize.
  */
 struct amdgpu_display_manager {