docs: fix incorrect description

Author: kenjis

user_guide_src/source/incoming/message.rst

@@ -151,8 +151,8 @@ Class Reference
         :returns: The current HTTP protocol version
         :rtype: string
 
-        Returns the message's current HTTP protocol. If none has been set, will return ``null``.
-        Acceptable values are ``1.0``, ``1.1`` and ``2.0``.
+        Returns the message's current HTTP protocol. If none has been set, will
+        return ``1.1``.
 
     .. php:method:: setProtocolVersion($version)