Update documentation

docs/rules/arrow-body-style.md

@@ -4,14 +4,17 @@ Arrow functions can omit braces when there is a single statement in the body. Th
 
 ## Rule Details
 
-This rule can enforce the use of braces around arrow function body.
+Arrow functions have two syntactic forms for their function bodies.  They may be defined with a *block* body (denoted by curly braces) `() => { ... }` or with a single expression `() => ...`, whose value is implicitly returned.
+
+This rule may be used to standardize your usage and thus avoid syntactic ambiguity.
 
 ## Options
 
 The rule takes one option, a string, which can be:
 
 * `"always"` enforces braces around the function body
 * `"as-needed"` enforces no braces where they can be omitted (default)
+* `"never"` enforces no braces around the function body (constrains arrow functions to the role of returning an expression)
 
 ### "always"
 
@@ -84,3 +87,30 @@ let foo = () => {
     // do nothing.
 };
 ```
+
+### "never"
+
+When the rule is set to `"never"` the following patterns are considered problems:
+
+```js
+/*eslint arrow-body-style: ["error", "never"]*/
+/*eslint-env es6*/
+
+let foo = () => {
+    return 0;
+};
+let foo = (retv, name) => {
+    retv[name] = true;
+    return retv;
+};
+```
+
+The following patterns are not considered problems:
+
+```js
+/*eslint arrow-body-style: ["error", "as-needed"]*/
+/*eslint-env es6*/
+
+let foo = () => 0;
+let foo = () => ({ foo: 0 });
+```