gdb/doc: revert previous vforkdone change

gdb/doc: revert previous vforkdone change

The previous manual change was wrong.  The vfork parent thread ID
should be reported with the usual "thread" magic register:

   Sending packet: $vCont;c:p7260.7260#1e...Packet received: OK
 -   Notification received: Stop:T05vforkdone:;
 +   Notification received: Stop:T05vforkdone:;thread:p7260.7260
                                               ^^^^^^^^^^^^^^^^^

This is already how the parent is reported in the vfork/fork events,
and is actually what the fix made gdbserver do.  Following the
documentation change, the event would have been reported like this
instead:

    Notification received: Stop:T05vforkdone:p7260.7260

gdb/doc/ChangeLog:
2015-09-15  Pedro Alves  <palves@redhat.com>

	PR remote/18965
	* gdb.texinfo (Stop Reply Packets): Revert previous change to
	the vforkdone description.