message: Clarify opentype tag format
diff: @@ -47,10 +47,15 @@ let long_date_time : Format = s64be;
  /// Array of four `U8`s used to identify a table, design-variation axis, script,
  /// language system, feature, or baseline.
  ///
  +/// The elements of the array are expected to be in the range [0x20, 0x7E].
  +/// This corresponds to the range of printable ASCII characters.
  +///
  /// ## References
  ///
  /// - [Microsoft's OpenType Spec: Tag](https://docs.microsoft.com/en-us/typography/opentype/spec/otff#dt_Tag)
  -let tag : Format = array8 4 u8;
  +let tag : Format =
  + // TODO: constrain array elements to the range 0x20 to 0x7E.
  + array8 4 u8;
  /// A format that consumes no input.
  let empty : Format = {};
  