Mention that @#chan/+#chan requires CHW capab.

doc/technical/ts6-protocol.txt

@@ -542,6 +542,7 @@ The target can be:
   remotely)
 - a status character ('@'/'+') followed by a channel name, to send to users
   with that status or higher only.
+  capab: CHW
   propagation: all servers with -D users with appropriate status
 - a user@server message, to send to users on a specific server. The exact
   meaning of the part before the '@' is not prescribed, except that "opers"