Prevent future errors in docs from going unnoticed

This is the same approach as used in stdlib https://github.com/rust-lang/rust/blob/e326e86b47edcded3313944a81f126a633032d86/src/libstd/lib.rs#L221
which I discovered via rust-lang/rust#41574.

Author: bradleybeddoes

src/lib.rs

@@ -10,6 +10,7 @@
 //! [mio]: https://github.com/carllerche/mio
 
 #![warn(missing_docs)]
+#![doc(test(no_crate_inject, attr(deny(warnings))))]
 // TODO: Remove this when it's a hard error by default (error E0446).
 // See Rust issue #34537 <https://github.com/rust-lang/rust/issues/34537>
 #![deny(private_in_public)]