gst/playback/: Cleanups and some more documentation.

gst/playback/: Cleanups and some more documentation.

Original commit message from CVS:
* gst/playback/README:
* gst/playback/gstdecodebin.c: (close_pad_link), (try_to_link_1):
* gst/playback/gstplaybin.c: (gst_play_bin_init),
(gst_play_bin_dispose), (gst_play_bin_set_property),
(remove_sinks), (setup_sinks), (gst_play_bin_change_state),
(gst_play_bin_get_event_masks), (gst_play_bin_send_event),
(gst_play_bin_get_formats), (gst_play_bin_convert),
(gst_play_bin_get_query_types), (gst_play_bin_query):
Cleanups and some more documentation.