Clarify `rustc-args` vs `rustdoc-args` in cross-compiling section

[Nemo157]

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.

[ChrisDenton]

I think the sentence after that also makes it sound more generally applicable then it is:

Here, the compiler arguments are set so that #[cfg(docsrs)] (not to be confused with #[cfg(doc)]) can be used for conditional compilation.

Which I think may have confused me (though I don't quite remember). EDIT: In any case it would be good to have the standard practice documented somewhere on that page.

[syphar]

I assume after #2390 this should be clear enough?

Closing this issue, feel free to reopen when not.