Test rustc output blocks in src/doc/rustdoc/src/ documentation

[joshtriplett]

Per discussion with @jyn514 on Zulip: this and other rustdoc documentation files should have the output they contain tested. #80527 turned up a case where the output didn't match the compiler behavior.

[ehuss]

Can you clarify what you mean by "doctests"? All book examples are tested, I'm guessing you mean something like "compiletest" or lint-docs?

I think the lint-docs tool could be extended to also generate the rustdoc chapter (it currently explicitly ignores rustdoc lints), though the format would need to change somewhat and the documentation would need to move to the source code.

If you're not familiar with lint-docs, there's a tiny amount of documentation here. I realize now there aren't any high-level docs, I should probably add a README to it.

[joshtriplett]

@ehuss The issue is that the output sections aren't being checked, which is why they were able to mismatch rustc's output without producing a test failure.

Updated the issue to clarify.