NLL: document specs for (new) semantics in rust ref (incl. deviations from RFC)

[pnkfelix]

The NLL RFC provided a specification for what we planned to implement. (Or at least it tried to do so.)

Since then, the NLL implementation made something that deviated in various ways from that specification.

This ticket is just noting that:

1. we did deviate in various ways, and
2. we should plan to document the actual semantics, in a manner suitable for the rust reference.

It would be good to link to here any PRs/issues where such deviations were implemented or discussed

[nikomatsakis]

Here are some things I would like to see if a final Rust Reference addition:

• Describing our handling of match, which turned out to be a significant complication (see e.g. this comment
• Describing how the move analysis works
  • In particular perhaps some of the limitations around handling structs and things?
• Describing the special treatment of Box
• "Dangly paths", however that winds up being resolved

[nikomatsakis]

Marking as deferred as this is not an edition blocker.

[pnkfelix]

Re-triaging for #56754. P-medium.

[steveklabnik]

Triage: this issue is on the rust repo, but is tracking work for the reference.