docs/: Small docs updates.

docs/: Small docs updates.

Original commit message from CVS:
* docs/design/part-events.txt:
* docs/design/part-gstpipeline.txt:
* docs/design/part-messages.txt:
* docs/design/part-overview.txt:
* docs/design/part-seeking.txt:
* docs/design/part-states.txt:
* docs/design/part-trickmodes.txt:
* docs/manual/advanced-position.xml:
Small docs updates.

* gst/gstobject.h:
People think !! is ugly, this looks better.

* gst/gstpad.c: (gst_pad_set_blocked_async):
Remove !! since it's fixed elsewhere now.