Document that timeoutadd accepts negative values

Both timeoutset and timeoutadd accept negative value for each part of their
time argument (hours, minutes, seconds). I think using this feature shouln't be
encouraged as it makes the time less intuitive, but I see no reason to add more
code to prevent it.

Still, documenting that timeoutadd can be used to reduce the timeout is
important, so mention that it accepts negative values. Creative users may
discover that 1:-10 is 50 seconds by themselves.

See #1272.

server/commands.cpp

@@ -447,7 +447,7 @@ static struct command commands[] = {
         "more time to finish their actions.\n"
         "The time is specified as hours, minutes, and seconds using the "
         "format hh:mm:ss (minutest and hours and are "
-        "optional)."),
+        "optional). Negative values are allowed."),
      nullptr, CMD_ECHO_ALL, VCF_NONE, 50},
     {"timeoutincrease", ALLOW_CTRL,
      // TRANS: translate text between <> only