[media] media: Improve documentation for link_setup/link_modify

Those callbacks are called with the media_device.graph_mutex held.

Add a note about that, as the code called by those notifiers should
not be touching in the mutex.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>