UNWIND_NULL_ID is no longer used anywhere. Update comments.

UNWIND_NULL_ID is no longer used anywhere.  Update comments.

Unfortunately, UNWIND_NULL_ID is exported to Python as
gdb.FRAME_UNWIND_NULL_ID so we can't really eliminate it.

(I'd assume scripts just check the result of Frame.unwind_stop_reason,
and compare it to gdb.FRAME_UNWIND_NO_REASON.  That at most, they'll
pass the result of Frame.unwind_stop_reason to
gdb.frame_stop_reason_string.  I'd prefer to just get rid of it, but
because we make an API promise, we get to keep this around for
compatibility, in case a script does refer to gdb.FRAME_UNWIND_NULL_ID
directly.)

gdb/
2013-11-29  Pedro Alves  <palves@redhat.com>

	* unwind_stop_reasons.def (UNWIND_NULL_ID): Update comment.

gdb/doc/
2013-11-29  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Frames In Python) <gdb.FRAME_UNWIND_NULL_ID>:
	Update comment.