improved help message for rustdoc::broken_intra_doc_links

[lolbinarycat]

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