Doc updates.

Doc updates.

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gst.c: (gst_init_get_option_group), (gst_init_check),
(ensure_current_registry_forking), (ensure_current_registry),
(parse_one_option), (parse_goption_arg), (gst_deinit),
(gst_registry_fork_is_enabled), (gst_registry_fork_set_enabled):
* gst/gst.h:
Doc updates.
Added API and command line option to disable registry forking in
addition to the environment variable.
Constify some static arrays.
Added some more debug.
Don't deinit twice.
API: gst_registry_fork_is_enabled()
API: gst_registry_fork_set_enabled()
API: --gst-disable-registry-fork command line option