[guide] Add explanation for semicolon enforcement rule

Also update code samples to highlight ways in which ES6
currently mishandles code when relying on Automatic Semicolon
Insertion

Revise hyperlink and example

- Use more up-to-date TC39 reference page on github.io
- wrap returnless function with side-effects in curly braces

also, clean up punctuation on a long comment line

Always use single-quoted strings

Except for when the string needs to be double-quoted because the string contains apostrophes

Update second example to incorporate IIFE

An IIFE is a more realistic example of code that developers may encounter which would raise an exception when relying completely upon ASI

README.md

@@ -2767,24 +2767,45 @@ Other Style Guides
   <a name="semicolons--required"></a><a name="20.1"></a>
   - [21.1](#semicolons--required) **Yup.** eslint: [`semi`](https://eslint.org/docs/rules/semi.html) jscs: [`requireSemicolons`](http://jscs.info/rule/requireSemicolons)
 
+    > Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called [Automatic Semicolon Insertion](https://tc39.github.io/ecma262/#sec-automatic-semicolon-insertion) to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.
+
     ```javascript
-    // bad
-    (function () {
-      const name = 'Skywalker'
-      return name
-    })()
+    // bad - raises exception
+    const luke = {}
+    const leia = {}
+    [luke, leia].forEach(jedi => jedi.father = 'vader')
+
+    // bad - raises exception
+    const reaction = "No! That's impossible!"
+    (async function meanwhileOnTheFalcon(){
+      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
+      // ...
+    }())
+
+    // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
+    function foo() {
+      return
+        'search your feelings, you know it to be foo'
+    }
 
     // good
-    (function () {
-      const name = 'Skywalker';
-      return name;
+    const luke = {};
+    const leia = {};
+    [luke, leia].forEach((jedi) => {
+      jedi.father = 'vader';
+    });
+
+    // good
+    const reaction = "No! That's impossible!";
+    (async function meanwhileOnTheFalcon(){
+      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
+      // ...
     }());
 
-    // good, but legacy (guards against the function becoming an argument when two files with IIFEs are concatenated)
-    ;((() => {
-      const name = 'Skywalker';
-      return name;
-    })());
+    // good
+    function foo() {
+      return 'search your feelings, you know it to be foo';
+    }
     ```
 
     [Read more](https://stackoverflow.com/questions/7365172/semicolon-before-self-invoking-function/7365214#7365214).