serde: Update base64.rs with a comment

Some minimal comment after https://github.com/ruma/ruma/pull/773#discussion_r757822969.

crates/ruma-serde/src/base64.rs

@@ -9,6 +9,7 @@ pub struct Base64<B = Vec<u8>> {
     bytes: B,
 }
 
+// See https://github.com/matrix-org/matrix-doc/issues/3211
 const BASE64_CONFIG: base64::Config = base64::STANDARD_NO_PAD.decode_allow_trailing_bits(true);
 
 impl<B: AsRef<[u8]>> Base64<B> {