Updating can-define/map documentation

docs/deleteKey.md

@@ -5,17 +5,45 @@
 
 @signature `map.deleteKey(key)`
 
-Deletes a key that was added to an instance, but not pre-defined by the type.
+  Deletes a key that was added to an instance, but not pre-defined by the type.
+
+  ```js
+  import {DefineMap, Reflect} from "can";
+
+  const map = new DefineMap( {propA: "valueA"} );
+  console.log( map.propA ); //-> "valueA"
+
+  map.deleteKey("propA");
+  console.log( map.propA ); //-> undefined
+  ```
+  @codepen
+
+  @param {String} key A string of the key being deleted.
+
+  @return {can-define/map/map} The map instance for chaining.
+
+@body
+
+## Use
+
+If `deleteKey` is called on a pre-defined type it sets the value to `undefined`. This is to keep the setters and getters on all instances of that Type.
 
 ```js
-import {DefineMap} from "can";
+import {DefineMap, Reflect} from "can";
 
-var Type = DefineMap.extend({seal: false},{
-  propA: "string"
-});
+const Type = DefineMap.extend(
+  {seal: false},
+  {
+    propA: "string"
+  }
+);
 
-var map = new Type({propA: "valueA"});
+const map = new Type( {propA: "valueA"} );
 map.set("propB","valueB");
-map.deleteKey("propB"); // this works.
-map.deleteKey("propA"); // this does not work.
+map.deleteKey("propB"); // This works.
+map.deleteKey("propA"); // This doesn't delete the key, but does set the key to undefined.
+
+console.log( Reflect.getOwnKeys(map) ); //-> ["propA"]
+console.log( map.propA ); //-> undefined
 ```
+@codepen