split up method binding and add description for memoization assignment operator

doc/playground.md

@@ -18,11 +18,19 @@ to5.transform("code", { playground: true });
 
  * [Memoization operator](#memoization-operator)
  * [Method binding](#method-binding)
+ * [Method binding function shorthand](#method-binding-function-shorthand)
  * [Object getter memoization](#object-getter-memoization)
  * [This shorthand](#this-shorthand)
 
 ### Memoization assignment operator
 
+The memoization assignment operator allows you to lazily set an object property.
+It checks whether there's a property defined on the object and if there isn't then
+the right hand value is set.
+
+This means that `obj.x` in the following `var x = { x: undefined }; obj.x ?= 2;`
+will still be `undefined` because it's already been defined on the object.
+
 ```javascript
 var obj = {};
 obj.x ?= 2;
@@ -42,7 +50,7 @@ var obj = {};
 obj.x ?= 2;
 ```
 
-equivalent to:
+equivalent to
 
 ```javascript
 var obj = {};
@@ -54,17 +62,25 @@ if (!Object.prototype.hasOwnProperty.call(obj, "x")) obj.x = 2;
 ```javascript
 var fn = obj#method;
 var fn = obj#method("foob");
-
-["foo", "bar"].map(#toUpperCase); // ["FOO", "BAR"]
-[1.1234, 23.53245, 3].map(#toFixed(2)); // ["1.12", "23.53", "3.00"]
 ```
 
-equivalent to:
+equivalent to
 
 ```javascript
 var fn = obj.method.bind(obj);
 var fn = obj.method.bind(obj, "foob");
+```
 
+### Method binding function shorthand
+
+```javascript
+["foo", "bar"].map(#toUpperCase); // ["FOO", "BAR"]
+[1.1234, 23.53245, 3].map(#toFixed(2)); // ["1.12", "23.53", "3.00"]
+```
+
+equivalent to
+
+```javascript
 ["foo", "bar"].map(function (val) { return val.toUpperCase(); });
 [1.1234, 23.53245, 3].map(function (val) { return val.toFixed(2); });
 ```