Rollup merge of #124870 - Lokathor:update-result-docs, r=dtolnay

Update Result docs to the new guarantees

The `Option` docs already explain the guarantees given in [RFC 3391](https://github.com/rust-lang/rfcs/blob/master/text/3391-result_ffi_guarantees.md), so all that we need is a paragraph saying that some `Result` type combinations will also qualify.

Part of rust-lang/rust#110503