Need to document how method resolution ambiguties are/aren't resolved

[pnkfelix]

Location

• The Reference: https://github.com/rust-lang/reference/issues

Summary

Spawned off of #81211 (comment)

We need better documentation of the method resolution rules, in terms of how ambiguities are resolved (to either a single method or to a method ambiguity error).

I wrote tests in PR #81294 that tried to enumerate the relevant combinations; see:

• https://github.com/rust-lang/rust/blob/master/src/test/ui/derives/derive-Debug-use-ufcs-struct.rs
• https://github.com/rust-lang/rust/blob/master/src/test/ui/derives/derive-Debug-use-ufcs-tuple.rs

But we still need better docs in the reference

[pnkfelix]

oops this doesn't belong here;

filed under rust-lang/reference#1308 instead