Add IE 11 doc note to addMapEntry. [ci skip]

lodash.js

@@ -381,6 +381,7 @@
    * @returns {Object} Returns `map`.
    */
   function addMapEntry(map, pair) {
+    // Don't return `Map#set` because it doesn't return the map instance in IE 11.
     map.set(pair[0], pair[1]);
     return map;
   }