Enable the cfg_doc feature on docs.rs

[Michael-F-Bryan]

Describe the bug

The wasmer crate uses conditional compilation to add/remove a lot of functionality from the public API, but at the moment there's no easy way to tell what is available when you check out the docs. Rustdoc actually has a doc_cfg feature which will show a message like "Available on crate feature fs only".

Solution

We can fix this pretty easily by updating the wasmer crate's lib.rs.

#![cfg_attr(docsrs, feature(doc_cfg))]

(from the lib.rs in tokio)