message: docs(headers): Explain content_type assigned type
  Explain why content_type mime params are not generic over slice.
  Related to
diff: @@ -12,6 +12,10 @@ header! {
  /// within the scope of the received message semantics, after any content
  /// codings indicated by Content-Encoding are decoded.
  ///
  + /// Although the `mime` crate allows the mime options to be any slice, this crate
  + /// forces the use of Vec. This is to make sure the same header can't have more than 1 type. If
  + /// this is an issue, it's possible to implement `Header` on a custom struct.
  + ///
  /// # ABNF
  /// ```plain
  /// Content-Type = media-type
  