bootstrap: document how to reproduce broken intra-doc links locally

[peter-lyons-kehl]

Neither x test tidy --bless, nor x test tidy --doc, report incorrect links from /// rustdocs. Example:

1. Incorrect/unregistered markdown link shortcut, and/or broken intradoc code link:
   https://github.com/rust-lang/rust/actions/runs/4429807104/jobs/7770725259#step:26:2652

     |     /// Like [from_iter], but coallocation-aware.
     |               ^^^^^^^^^ no item named `from_iter` in scope
     |
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
     = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`

2. URLs not made into links:
   https://github.com/rust-lang/rust/actions/runs/4429807104/jobs/7770725259#step:26:2704

49 | /// Workaround https://github.com/rust-lang/rust/issues/108751
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://github.com/rust-lang/rust/issues/108751>`

Please, either

• make (local) tidy report the above, or
• add steps (to the documentation) on how to check the above locally.

Side notes

• the commits for the above PR will most likely disappear as I rebase & force push, but hopefully the CI log will stay for a little while.
• I've searched in issues & https://www.google.com/search?q=site%3Ahttps%3A%2F%2Fgithub.com%2Frust-lang%2Frust+tidy+%22unresolved+link+to%22, and I couldn't find any similar issues.

Thank you in advance.

[peter-lyons-kehl]

I see, it's x doc library/alloc, or ./x.py doc library/alloc (or similar for other libraries etc.).

Unfortunately, that is not obvious. I'll dig around rustc-dev-guide for where this could be mentioned more.

[clubby789]

@rustbot label +T-bootstrap