clarify label syntax

[bnewbold]

This should be a quick fix, just updating the hosting docs and UI a bit; in a rush so don't have time to do a PR at the moment.

Basically we should clarify that the client app has a lot of additional restrictions on label syntax, and reference the "preferred" syntax in the specs being essentially required.

This caused a long debug flow for one ozone hoster: bluesky-social/atproto#2511

[itaru2622]

@bnewbold which is better for maintainers?

• add below URL with shortest description in needed places.
  https://atproto.com/specs/label#recommended-string-syntax
• add(copy) content of above and URL in needed places.