gdb: Document vMustReplyEmpty remote packet

Add mention of the vMustReplyEmpty to the remote serial protocol
documentation.  It is important that this packet be treated in the same
fashion as any other unknown 'v' packet, and I have tried to reflect
this in the description of the packet, it is not simply the case that we
_must_ return the empty string for this packet.

As the intention is that we should treat this packet as unknown then an
argument could be made that we should not document it, however, for
someone implementing a gdbserver from scratch, seeing an undocumented
packet arrive from gdb is confusing, and will probably cause them to
have to read the code in order to check how this packet should be
handled, which is not ideal.

gdb/doc/ChangeLog:

	* gdb.texinfo (Packets): Document vMustReplyEmpty packet.

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