Expand and organize offset_of! documentation.

[kpreid]

• Give example of how to get the offset of an unsized tail field (prompted by discussion Expand CloneToUninit documentation. #133055 (comment)).
• Specify the return type.
• Add section headings.
• Reduce “Visibility is respected…”, to a single sentence.
• Move offset_of_enum documentation to unstable book (with link to it).
• Add offset_of_slice documentation in unstable book.

r? Mark-Simulacrum

[jieyouxu]

Suggestion: maybe link to the tracking issue?

[`offset_of_enum`]: https://github.com/rust-lang/rust/issues/120141

But also, this is describing the interaction of this stable macro w/ an unstable feature. Maybe move this to a section like

# Interactions with unstable features

or maybe move this bit to docs for offset_of_enum? 🤔

[Mark-Simulacrum]

I think it's OK in this particular case to reference unstable features, even if we normally don't, since this seems like a natural place for people to look for that functionality. I don't think we have offset_of_enum! (yet?) - not sure what you mean by that.

[jieyouxu]

Right.

I don't think we have offset_of_enum! (yet?) - not sure what you mean by that.

Not sure why I wrote this lmao.

[kpreid]

I assumed you meant to put the documentation in the unstable book, and I think that makes sense. Should I not?

(Also while writing that documentation, I found a bug-or-design-question: #138327.)

[jieyouxu]

Right, I think that's what I wanted to say lol

[Mark-Simulacrum]

I think we should at minimum link to those docs, even if we don't have a stable thing to link to - otherwise people will handroll stuff I suspect and we've seen that cause UB, etc. in the past.

[jieyouxu]

IOW, probably do nothing here :D

[kpreid]

I've now moved all the unstable feature documentation to the unstable book, except for links to it from offset_of, and summaries.