I'm a Tokio maintainer. Here are the steps to do this:
First you add this to your Cargo.toml:
[package.metadata.docs.rs]
all-features = true
rustdoc-args = ["--cfg", "docsrs"]
This will make docs.rs compile your documentation with RUSTFLAGS="--cfg docsrs" set. We can use this to conditionally enable nightly features when building documentation, but not when building it normally.
Now you add this to your lib.rs file:
#![cfg_attr(docsrs, feature(doc_cfg))]
Then, on anything that needs such an annotation, you add the following
#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
This supports all of the syntax that #[cfg(...)] does, including stuff like platform-specifiers and any/all. Tokio often sets this annotation using the macros in src/macros/cfg.rs. Note that the annotation works recursively, so putting it on a module also affects the stuff inside (sometimes, it's a bit flaky).
Finally, when building the documentation locally, you run the following command:
RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features