doc: update http.protocol description

doc/userguide/rules/http-keywords.rst

@@ -244,6 +244,15 @@ Example::
 
     alert http any any -> any any (flow:to_server; http_protocol; content:"HTTP/1.0"; sid:1;)
 
+``http.protocol`` replaces the previous keyword name: ```http_protocol``. You may continue
++to use the previous name, but it's recommended that rules be converted to use
++the new name.
+
+Example::
+
+    alert http any any -> any any (flow:to_server; http.protocol; content:"HTTP/1.0"; sid:1;)
+
+
 http_request_line
 -----------------