gstreamer: segment: small docs addition

[Tim-Philipp Müller]

On Sun, 2015-04-05 at 11:59 -0400, Nicolas Dufresne wrote:

> This addition isn't fully correct. Positon is also used by
> gst_segment_do_seek().

I'm quite aware of that, but as I see it do_seek is also used internally
by elements then. What I meant by the comment is that it shouldn't be
considered as state that's signalled through the pipeline via the
SEGMENT event.

 -Tim