[RFC17] precise prefix conventions

rfc/0017_transaction_comment_encryption.md

@@ -40,7 +40,11 @@ The maximum length of a message is 173 bytes.
 
 ### Prefix
 
-A constant value that indicates the type of format. For the present format defined in this RFC, the prefix is `0x0100`. A possible future new format should choose a different prefix. This prefix can also be incremented for a future version of the present format.
+A constant value that indicates the type of format. For the present format defined in this RFC, the prefix is `0x0100`.
+
+Future version of the present format must increment second byte (next version use prefix `0x0101`).
+
+A possible future format should increment the first byte of the prefix.
 
 ### Message type