[NNTP] Consistency in replies to 420 in RFC 3977

Following a discussion with Alfred Hönes, who reported:

> Appendix C, on page 119, says:
> 
>     Response code 412
>        Generated by: ARTICLE, BODY, GROUP, HDR, HEAD, LAST, LISTGROUP,
>        NEXT, OVER, STAT
>        Meaning: no newsgroup selected.
> 
>     Response code 420
>        Generated by: ARTICLE, BODY, HDR, HEAD, LAST, NEXT, OVER, STAT
>        Meaning: current article number is invalid.
> 
> Please be aware of the different wording used:
>        "no newsgroup selected" vs. "current article number is invalid"
> 
> These two phrases, respectively, are used in a totally consistent way,
> throughout the RFC, in the specification of possible command responses
> when it comes to referring to these two response codes.
>
> The text, "no newsgroup selected", also appears in all examples
> where response code 412 is returned by the server.
> 
> Interestingly enough, all examples in the RFC where the response
> code 420 is returned by the server consistently do *not* use the
> above quoted phrase from Appendix C, but instead say:
> 
>        [S] 420 No current article selected

Well seen