nth and only-tags are documented but not yet in a release

[sogaiu]

There was some confusion reported at Zulip as to the availability of nth and only-tags. nth and only-tags already have documentation at the website even though they are not available in a release.

Since there are docs for many versions of Janet, it seems nicer if what does appear in the docs for a given version really is part of that release.

Don't know if this is a significant issue, but thought it might be worth noting for future activities (^^;