Mention IsomorphicString and update ByteString guidance appropriately

[aphillips]

In w3ctag/design-principles#454 there is a discussion with @annevk about the current guidance related to ByteString. We should update our guidance to be clearer and more thorough about the usage of byte-array/byte-string types in protocols and point to isomorphic string handling explicitly. See his/my comments of 2025-02-13/14