media: vimc: stream: add missing function documentation

Add comments at vimc_streamer_s_stream and vimc_streamer_thread, making
the vimc-stream totally documented.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
[hverkuil-cisco@xs4all.nl: fix typo: in a fixed framerate -> at a fixed framerate]
[hverkuil-cisco@xs4all.nl: fix typo: stops the thread -> stop the thread]
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>