gst/: Small doc updates.

gst/: Small doc updates.

Original commit message from CVS:
* gst/gstbuffer.c:
* gst/gsttask.c: (gst_task_join):
Small doc updates.

* gst/gstpad.c: (gst_pad_activate_pull), (gst_pad_activate_push),
(gst_pad_stop_task):
When pad (de)activation failed for some reason, restore the old
activation mode and set the pad to flushing instead of assuming the
pad is deactivated.
If the _task_join() failed, reinstall the task on the pad so that it can
be stopped later and return an error.