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.

1 files changed, 13 insertions, 0 deletions

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index f2e6156..9fb70f6 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -35827,6 +35827,19 @@ for an error
 for success
 @end table
 
+@item vMustReplyEmpty
+@cindex @samp{vMustReplyEmpty} packet
+The correct reply to an unknown @samp{v} packet is to return the empty
+string, however, some older versions of @command{gdbserver} would
+incorrectly return @samp{OK} for unknown @samp{v} packets.
+
+The @samp{vMustReplyEmpty} is used as a feature test to check how
+@command{gdbserver} handles unknown packets, it is important that this
+packet be handled in the same way as other unknown @samp{v} packets.
+If this packet is handled differently to other unknown @samp{v}
+packets then it is possile that @value{GDBN} may run into problems in
+other areas, specifically around use of @samp{vFile:setfs:}.
+
 @item vRun;@var{filename}@r{[};@var{argument}@r{]}@dots{}
 @cindex @samp{vRun} packet
 Run the program @var{filename}, passing it each @var{argument} on its