You can configure how your crate is built by adding package metadata to your Cargo.toml, e.g.:

[package.metadata.docs.rs]
rustc-args = ["--cfg", "docsrs"]

—https://docs.rs/about/builds#cross-compiling

This may be a little confusing compared to the standard practice of rustdoc-args = ["--cfg", "docsrs"], this section should clarify that this cfg is used for things like the build.rs or dependencies but not for the docs themselves.