Set doc(cfg(feature = "std")) on std trait impls

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/error_impls.rs

@@ -5,6 +5,7 @@
 // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+#![cfg_attr(docsrs, doc(cfg(feature = "std")))]
 extern crate std;
 
 use crate::Error;