cargo doc error in using projects

I've just filed https://github.com/rust-lang/rust/issues/71908 about an error from `cargo doc --no-deps` which is ultimately reporting an issue coming from async-std. I'm not entirely sure what is triggering it yet :/.

That bug is about the quality of the error; this bug is about fixing the block, presumably in the macro generated output, or something - I haven't tracked down what I"m using that is triggering it as I mentioned, so I can't be certain of the context yet.

```
warning: doc comment contains an invalid Rust code block
--> /home/robertc/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.5.0/src/utils.rs:243:42
|
243 | $($head)* extension_trait!(@ext (#[doc = $d]) $($tail)*);
| ^^^^^^^^^^^
|
= help: mark blocks that do not contain Rust code as text: ```text
```