[media] DocBook: Document AUDIO_CONTINUE ioctl

Although this ioctl is only used at the av7110 driver, it is not
described at the API docbook. Yet, AUDIO_PAUSE ioctl description
somewhat describes it. Fill the gap by using the information there
and by looking inside av7110 implementation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>