added code for downstream events, reviewed docs in gstevent.c

added code for downstream events, reviewed docs in gstevent.c

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gstbin.c: (bin_element_is_src), (src_iterator_filter),
(gst_bin_iterate_sources), (gst_bin_send_event):
* gst/gstbin.h:
* gst/gstelement.c: (gst_element_send_event):
* gst/gstevent.c:
* gst/gstpad.c: (gst_pad_send_event):
added code for downstream events, reviewed docs in gstevent.c