Fix pipeline doc: Warn on unexpected cfgs instead of error (#276)

* Fix pipeline doc: Warn on unexpected cfgs instead of error

* Allow unexpected cfg and fix fmt

Author: NobodyXu

src/lib.rs

@@ -29,6 +29,7 @@
 //! ---------|------
 // TODO: Kill rustfmt on this section, `#![rustfmt::skip::attributes(cfg_attr)]` should do it, but
 // that's unstable
+#![allow(unexpected_cfgs)]
 #![cfg_attr(
     feature = "futures-io",
     doc = "[`futures-io`](crate::futures) | [`futures::io::AsyncBufRead`](futures_io::AsyncBufRead), [`futures::io::AsyncWrite`](futures_io::AsyncWrite)"