Cargo.toml: Consider building docs with all features

[mxinden]

Consider adding the below to your Cargo.toml to have all features documented on docs.rs:

[package.metadata.docs.rs]
all-features = true

https://github.com/libp2p/rust-libp2p/blob/6cc3b4ec52c922bfcf562a29b5805c3150e37c75/Cargo.toml#L68-L69

This might be a deliberate choice. Feel free to ignore.