Rollup merge of #82651 - jyn514:rustdoc-warnings, r=GuillaumeGomez

Cleanup rustdoc warnings

## Clean up error reporting for deprecated passes

Using `error!` here goes all the way back to the original commit, https://github.com/rust-lang/rust/pull/8540. I don't see any reason to use logging; rustdoc should use diagnostics wherever possible. See https://github.com/rust-lang/rust/pull/81932#issuecomment-785291244 for further context.

- Use spans for deprecated attributes
- Use a proper diagnostic for unknown passes, instead of error logging
- Add tests for unknown passes
- Improve some wording in diagnostics

##  Report that `doc(plugins)` doesn't work using diagnostics instead of `eprintln!`

This also adds a test for the output.

This was added in https://github.com/rust-lang/rust/pull/52194. I don't see any particular reason not to use diagnostics here, I think it was just missed in https://github.com/rust-lang/rust/pull/50541.

compiler/rustc_passes/src/check_attr.rs

@@ -576,7 +576,8 @@ impl CheckAttrVisitor<'tcx> {
                             sym::masked,
                             sym::no_default_passes, // deprecated
                             sym::no_inline,
-                            sym::passes, // deprecated
+                            sym::passes,  // deprecated
+                            sym::plugins, // removed, but rustdoc warns about it itself
                             sym::primitive,
                             sym::spotlight,
                             sym::test,