Document that Authenticator does the Base64 encoding and decoding.

src/authenticator.rs

@@ -1,5 +1,16 @@
 /// This will allow plugable authentication mechanisms.
+///
+/// This trait is used by `Client::authenticate` to [authenticate
+/// using SASL](https://tools.ietf.org/html/rfc3501#section-6.2.2).
 pub trait Authenticator {
+    /// Type of the response to the challenge. This will usually be a
+    /// `Vec<u8>` or `String`. It must not be Base64 encoded: the
+    /// library will do it.
     type Response: AsRef<[u8]>;
+    /// For each server challenge is passed to `process`. The library
+    /// has already decoded the Base64 string into bytes.
+    ///
+    /// The `process` function should return its response, not Base64
+    /// encoded: the library will do it.
     fn process(&self, &[u8]) -> Self::Response;
 }