gst/gstelement.*: Improve docs a little. Added Since: for new macro.

gst/gstelement.*: Improve docs a little. Added Since: for new macro.

Original commit message from CVS:
* gst/gstelement.c: (gst_element_message_full),
(gst_element_get_state_func):
* gst/gstelement.h:
Improve docs a little. Added Since: for new macro.
* gst/gstobject.c: (gst_object_sink):
* gst/gstpipeline.c: (gst_pipeline_change_state),
(gst_pipeline_set_new_stream_time):
* gst/gstpipeline.h:
Improve debugging and docs.
* gst/gstutils.c: (gst_element_state_change_return_get_name):
Improve debugging.