[media] docs-rst: media: better document refcount in struct dvb_frontend

The refcount field was added to the struct but it was not
properly documented.

Document it.

[mchehab@s-opensource.com: fix a merge conflict]

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>