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