[media] DocBook media: add revision entry for 3.15

[m.chehab@samsung.com: removed "Opening and Closing Devices" from the
 list of changes, as the patch with such change weren't applied]

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>