Update envelope.md

Co-authored-by: Mark Lodato <[email protected]>

envelope.md

@@ -32,20 +32,22 @@ the following form, called the "JSON envelope":
 
 See [Protocol](protocol.md) for a definition of parameters and functions.
 
-Arbitrary binary data can be encode in [Base64](https://tools.ietf.org/html/rfc4648),
-as a unicode string and enclosed in the envelope using the `payload` key.
-Either standard or URL-safe encoding is allowed.
-
-However, if the serialized payload body is a text string, it can *instead* be
-encoded in [UTF-8](https://tools.ietf.org/html/rfc3629) and enclosed in the
-envelope using the `payloadUtf8` key. The UTF-8 encoding has a lower overhead
-than Base64 for texts, and is more friendly for compression algorithms.
-
-Note:
-
-1. Use either the `payload` or `payloadUtf8` key in an envelope, not both;
-2. The choice of payload encoding does not impact
-   [the signing or the signatures](protocol.md#signature-definition).
+Exactly one of `payload` or `payloadUtf8` MUST be set:
+
+-   `payload` supports arbitrary SERIALIZED_BODY. 
+    [Base64Encode()](https://tools.ietf.org/html/rfc4648) transforms a byte
+    sequence to a Unicode string. Base64 has a fixed 33% space overhead but
+    supports payloads that are not necessarily valid UTF-8. Either standard or
+    URL-safe encoding is allowed.
+
+-   `payloadUtf8` only supports valid
+    [UTF-8](https://tools.ietf.org/html/rfc3629) SERIALIZED_BODY. `Utf8Decode()`
+    converts that UTF-8 byte sequence to a Unicode string. Regular JSON string
+    escaping applies, but this is usually more compact and amenable to
+    compression than Base64.
+
+Note: The choice of `payload` vs `payloadUtf8` does not impact the
+[the signing or the signatures](protocol.md#signature-definition).
 
 ### Multiple signatures