`ERR_INVALIDCAPCMD` not listed

[jwodder]

The Capability Negotiation specification states in part:

If a client sends a subcommand which is not in the list above or otherwise issues an invalid command, then numeric 410 (ERR_INVALIDCAPCMD) should be sent. The first parameter after the client identifier (usually nickname) should be the commandname; the second parameter should be a human-readable description of the error.

However, this reply is currently not listed on modern.ircdocs.horse, even though the CAP command is. It should probably be added.

[progval]

the CAP command is not documented here, only mentioned. IRCv3 already provides good documentation of the numeric so there is no need to duplicate it here.

[binhle113]