Some more documentation.

Some more documentation.

Original commit message from CVS:
* docs/design/part-element-sink.txt:
* docs/design/part-element-source.txt:
* gst/base/gstbasesink.c: (gst_basesink_class_init),
(gst_basesink_event), (gst_basesink_activate):
* gst/base/gstbasesink.h:
* gst/base/gstbasesrc.c: (gst_basesrc_init), (gst_basesrc_unlock),
(gst_basesrc_activate):
* gst/base/gstbasesrc.h:
* gst/gstelement.c: (gst_element_pads_activate):
Some more documentation.
Fixed scheduling decision in _pads_activate().