Rollup merge of #118006 - lcnr:discriminant-docs, r=compiler-errors

clarify `fn discriminant` guarantees: only free lifetimes may get erased

cc https://github.com/rust-lang/rust/pull/104299/files#r1397082347

don't think this necessitates a backport by itself, but should imo be included if one were to exist.

r? types

Author: matthiaskrgr