docs(headers): Explain content_type assigned type

Explain why content_type mime params are not generic over slice.

Related to #991

src/header/common/content_type.rs

@@ -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