media: docs-rst: Document EBUSY for VIDIOC_S_FMT

VIDIOC_S_FMT may return EBUSY if the device is streaming or there are
buffers allocated. Document this.

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