Suggest running gdbserver for a PID in container

Suggest running gdbserver for a PID in container

currently
	gdb -p <pid from a container>
will print:
	warning: Target and debugger are in different PID namespaces; thread lists and other data are likely unreliable

It correctly states the problem but it does not say how to solve it.

Originally I wanted to suggest also the Docker "-p 1234:1234" parameter but
I see the containers are more general topic than just Docker (even LxC etc.).

According to Gary future GDBs should be able to work even without gdbserver.
But currently gdbserver is still required.

gdb/ChangeLog
2016-03-17  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* linux-thread-db.c (check_pid_namespace_match): Extend the message.