Add suggestion for "ignore" doc code block

[GuillaumeGomez]

Part of #30032.

This PR adds a suggestion to help users when they have a "ignore" doc code block which is invalid rust code.

r? @jyn514

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
Checking which error codes lack tests...
Found 435 error codes
Found 0 error codes with no tests
Done!
tidy error: /checkout/src/test/rustdoc-ui/ignore-block-help.rs:3: unexplained "```ignore" doctest; try one:

* make the test actually pass, by adding necessary imports and declarations, or
* use "```text", if the code is not Rust code, or
* use "```compile_fail,Ennnn", if the code is expected to fail at compile time, or
* use "```should_panic", if the code is expected to fail at run time, or
* use "```no_run", if the code should type-check but not necessary linkable/runnable, or
* explain it like "```ignore (cannot-test-this-because-xxxx)", if the annotation cannot be avoided.

some tidy checks failed

[GuillaumeGomez]

Updated!

[jyn514]

r=me with or without fixing the nit

[jyn514]

@bors r+

[bors]

📌 Commit 7bc1eb4 has been approved by jyn514