Update core-ajax docs to reflect new auto behavior

core-ajax.html

@@ -20,7 +20,7 @@
         on-core-response="{{handleResponse}}"></core-ajax>
 
 With `auto` set to `true`, the element performs a request whenever
-its `url` or `params` properties are changed.
+its `url`, `params` or `body` properties are changed.
 
 Note: The `params` attribute must be double quoted JSON.