docs.rs doc build broken

[kpreid]

I noticed that https://docs.rs/async-executor/1.13.0 has a broken build. The docs.rs build logs say:

[INFO] [stderr] error[E0658]: `#[doc(cfg)]` is experimental
[INFO] [stderr]   --> src/lib.rs:61:20
[INFO] [stderr]    |
[INFO] [stderr] 61 | #[cfg_attr(docsrs, doc(cfg(any(feature = "static"))))]
[INFO] [stderr]    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stderr]    |
[INFO] [stderr]    = note: see issue #43781 <https://github.com/rust-lang/rust/issues/43781> for more information
[INFO] [stderr]    = help: add `#![feature(doc_cfg)]` to the crate attributes to enable

It appears that the problem is that #122 added a use of #[doc(cfg(...))] without also adding #![feature(doc_cfg)].