Properly set rustdoc attributes for docs.rs

[thomaseizinger]

On the topic of:

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

What this does is tell docs.rs to pass the docsrs conditional compilation flag to rustc, meaning any code that is guarded with cfg(docsrs) will now be included in the build.

We use this in the libp2p root crate to conditionally include the doc attribute:

rust-libp2p/src/lib.rs

Line 53 in 04b07c4

#[cfg_attr(docsrs, doc(cfg(feature = "deflate")))]

We need to do this conditionally because the #[doc(cfg)] attribute is not yet stable. docs.rs runs all their build pipelines on nightly so we can use it there.

Now, interestingly, we don't set this in our libp2p crate so all of these cfg_attr attributes have been doing nothing all this time :)

In this crate, it is used here: https://github.com/umgefahren/rust-libp2p/blob/6f4bca5bbfd141bc18d9d931c5d28b50a212f0fe/transports/onion/src/provider.rs#L31-L36

And will tell docs.rs to include a little "only available with feature tokio" tag next to the item.

Originally posted by @thomaseizinger in #2899 (comment)

[umgefahren]

I'm currently working on it, however it is a lot of effort, since I have to go through nearly every single line of code here.

I will make a pull request with the changes I applied to libp2p-core, so you can judge wether the changes are good.