docs/design/part-qos.txt: Bring docs in line with the code. Mostly the sign of the jitter was wrong in the docs.

Original commit message from CVS:
* docs/design/part-qos.txt:
Bring docs in line with the code. Mostly the sign of the jitter was
wrong in the docs.
* gst/gstclock.c:
Fix the docs for the jitter.
* gst/gstevent.c: (gst_event_new_custom), (gst_event_new_tag),
(gst_event_parse_tag), (gst_event_new_buffer_size),
(gst_event_parse_buffer_size), (gst_event_parse_qos),
(gst_event_new_seek), (gst_event_parse_seek),
(gst_event_new_navigation):
Make sure the GstStructure has no parent when creating custom
events.
Add some more argument checking so that we avoid 0.0 rates.
Flesh out the docs for the QoS event some more.