[media] DocBook: drop the word 'only'

There are already video output drivers that allow STREAMON without
any buffers queued, and with the change in vb2 there are now more
drivers like that.

So saying "The ioctl will succeed only when at least one output
buffer is in the incoming queue." isn't true. Just drop the word
'only'. We cannot say that it will also work if no output buffers are
queued as long as not all drivers are converted to vb2.

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