sasl: make docs.rs emit nice feature tags on feature-gated items

sasl/src/error.rs

@@ -5,6 +5,7 @@ use getrandom::Error as RngError;
 #[derive(Debug)]
 pub enum Error {
     #[cfg(feature = "scram")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "scram")))]
     /// An error while initializing the Rng.
     RngError(RngError),
     /// An error in a SASL mechanism.
@@ -12,6 +13,7 @@ pub enum Error {
 }
 
 #[cfg(feature = "scram")]
+#[cfg_attr(docsrs, doc(cfg(feature = "scram")))]
 impl From<RngError> for Error {
     fn from(err: RngError) -> Error {
         Error::RngError(err)