fix: add doc_cfg to improve docs.rs experience

[t3hmrman]

See comment in users.rust-lang.org -- on docs.rs it's very hard to tell where the tokio package comes from (the feature is required) -- so it's a good idea to use the recommended doc_cfg flag.

Thanks SkiFire13 for the recommendation

[t3hmrman]

Re-opening since the current documentation for runtime::tokio::wait_for still doesn't show the feature flag requirement