media: Documentation: Fix [GS]_ROUTING documentation

Add mention that successful VIDIOC_SUBDEV_G_ROUTING call will update
'num_routes' and remove mention about non-existing streams, which is
incorrect.

Fixes: ea73eda5 ("media: Documentation: Add GS_ROUTING documentation")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>