docs: design: fix some 0.10-isms in GstSegment docs

docs: design: fix some 0.10-isms in GstSegment docs

1) segment.accum -> segment.base

2) Refer to GstSegment members as S.foo instead of
NS.foo, the event is now called a segment event
rather than newsegment event.

3) There's no more abs_rate field in GstSegment,
and there never was an abs_applied_rate field.

https://bugzilla.gnome.org/show_bug.cgi?id=690564