Updated PROTOCOL.md

doc/PROTOCOL.md

@@ -133,6 +133,51 @@ having the following structure
     radio button `value`-attribute to be activated in `"value"`.
 	
 
+### Form Group Configuration Response
+
+As part of the server response, the JSON stream may contain a key
+`element-update`. This key stores information on how to modify HTML elements identified by `id`. Modifying in this 
+context refers to:
+
+ * Setting attribute values
+ * Deleting attributes
+ * Setting content of a HTML element.
+ 
+The content of `element-update` is outlined below
+
+    {
+        ...
+        "element-update" : {
+            "<element_id1>": {
+                "attr": {
+                    "<attr_name1>": "<value1>" | null,
+                    ...
+                    "<attr_nameN>": "<valueN>" | null
+                },
+                "content": "<element_content>"
+            },
+            ...
+            "<element_idN>": {
+                "attr": {
+                    "<attr_name1>": "<value1>" | null,
+                    ...
+                    "<attr_nameN>": "<valueN>" | null
+                },
+                "content": "<element_content>"
+            }
+        },
+        ...
+    }
+    
+The presence of `element-update` is optional. `<element_idN>` refers to the element's `id`-attribute value. It used 
+to uniquely identify the HTML element in the DOM. The properties `"attr"` and `"content"` are both optional.
+
+Supplying `null` as value for `"<attr_nameN>"` will remove the attribute from the HTLM element identified by 
+`"<element_idN>"`.
+
+If the element has no `"<attr_nameN>"` attribute, it will be created. In any case, the attribute's value will be set 
+to the value specified by `"<valueN>"`. See above for handling of `null` value.
+
 ### Redirection Response
 
 Depending on the request, the server may return redirection