Warn when rustdoc:: group is omitted from lint names

[jyn514]

When rustdoc lints were first made a tool lint, they gave an unconditional warning when you used the original name:

warning: lint `broken_intra_doc_links` has been renamed to `rustdoc::broken_intra_doc_links`
  --> $DIR/renamed-lint-still-applies.rs:2:9
   |
LL | #![deny(broken_intra_doc_links)]
   |         ^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `rustdoc::broken_intra_doc_links`
   |
   = note: `#[warn(renamed_and_removed_lints)]` on by default

That was reverted in #83203 because adding rustdoc::x lints would cause the code to break on old versions of the compiler (due to #66079 (comment), "fixed" in #83216 in the sense that you can now opt-in to not breaking on nightly, which is not ideal but register_tool is a long way from stabilizing). Since #80527 is now on 1.52.0 stable, we can re-enable the warning. For nightly users, they can change immediately and still have their code work on stable; for stable users, they can change their code in 12 weeks and still have it work up to 3 releases back (about 18 weeks). That seems reasonable to me.

r? @Manishearth cc @rust-lang/rustdoc

[Manishearth]

@bors r+

[bors]

📌 Commit 7a774a8 has been approved by Manishearth

[Mark-Simulacrum]

@bors rollup

(Just doing some testing don't mind me).

[bors]

⌛ Testing commit 7a774a8 with merge 64ae15d...

[bors]

☀️ Test successful - checks-actions
Approved by: Manishearth
Pushing 64ae15d to master...