Update envelope.md

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

Author: AdamZWu

envelope.md

@@ -18,12 +18,10 @@ the following form, called the "JSON envelope":
 
 ```jsonc
 {
-  // Arbitrary binary data can be encode in base64.
-  "payload": "<Base64(byte sequence)>",
-  // If the SERIALIZED_BODY is a text string,
-  // it can *instead* be encoded in UTF-8. 
-  // "payloadUtf8": "<Utf-8 encoded text>",
-
+  // Exactly one of the following must be set:
+  "payload": "<Base64Encode(SERIALIZED_BODY)>",
+  "payloadUtf8": "<Utf8Decode(SERIALIZED_BODY)>",
+  // End oneof
   "payloadType": "<PAYLOAD_TYPE>",
   "signatures": [{
     "keyid": "<KEYID>",