tonic: Unhide docs for Ascii and Binary, so Rustdoc shows various met…

…hods.

MetadataValue::from<i16> and other numeric types aren't showni n Rustdoc, because they're implemented for Metadata<Ascii>, and Ascii is hidden.

tonic/src/metadata/encoding.rs

@@ -51,11 +51,13 @@ pub trait ValueEncoding: Clone + Eq + PartialEq + Hash + self::value_encoding::S
     fn is_valid_key(key: &str) -> bool;
 }
 
+/// gRPC metadata values can be either ASCII strings or binary. Note that only
+/// visible ASCII characters (32-127) are permitted.
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
-#[doc(hidden)]
 pub enum Ascii {}
+
+/// gRPC metadata values can be either ASCII strings or binary.
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
-#[doc(hidden)]
 pub enum Binary {}
 
 // ===== impl ValueEncoding =====