docs/design/part-synchronisation.txt: Small updates.

docs/design/part-synchronisation.txt: Small updates.

Original commit message from CVS:
* docs/design/part-synchronisation.txt:
Small updates.
* gst/gstsegment.c: (gst_segment_set_seek),
(gst_segment_set_newsegment_full), (gst_segment_to_stream_time),
(gst_segment_to_running_time):
The seek format can be different from the segment format when the start
and stop values are not to be updated, when we only do a rate change for
example.
* tests/check/gst/gstsegment.c: (GST_START_TEST),
(gst_segment_suite):
Add a testcase for the rate-only seeks, checking that the format is
correctly ignored when start and stop are not updated.