improved help message for rustdoc::broken_intra_doc_links

Currently, the only suggestion rustdoc gives is to simply escape [ and ] with backslashes, or if there is a similarly-named item, to link to that instead.

there are a few other common errors that it could catch though:

• code snippets (like arr[idx+3]) should be surrounded by backticks instead of escaping each bracket individually
• if there is a similarly-named link reference definition, it should suggest referencing that (probable typo)
• if the type that is being linked to is outside the current documentation bundle (e.g. linking to a type in an alternative library for the purpose of comparison, or linking to a non-rust type), then it should recommend adding a link definition item.

inspired by discussion on #132748

Start with the rustdoc::broken_intra_doc_links diagnostic and the discussion in #132748. Review how its current escaping and similarly named-item suggestions are produced, then cover the three listed cases with appropriate guidance. Done means rustdoc offers targeted suggestions for code snippets, link reference definitions, and types outside the current documentation bundle.