Clarify language for the '?' packet

Clarify language for the '?' packet

Both QEMU and kgdb make the assumption that the '?' packet is only
sent during the initial setup of a gdbstub connection. Both use that
knowledge to reset breakpoints and ensure the gdbstub is in a
clean-state on a resumed connection. This can cause confusion for
others implementing clients that speak to gdbstub devices. To avoid
that make the language clearer that this is a start-up query packet
that you only expect to see once.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
gdb/doc/ChangeLog:

	* gdb.texinfo (Packets): Clarify language for ? packet.

Change-Id: Iae25d3110fe28b8d2467704962a6889e55224ca5