[guide] add some more justification for one-var and String() type coercions

Author: ljharb

README.md

@@ -590,7 +590,7 @@ Other Style Guides
   <a name="es6-rest"></a>
   - [7.6](#7.6) <a name='7.6'></a> Never use `arguments`, opt to use rest syntax `...` instead. [`prefer-rest-params`](http://eslint.org/docs/rules/prefer-rest-params)
 
-    > Why? `...` is explicit about which arguments you want pulled. Plus rest arguments are a real Array and not Array-like like `arguments`.
+    > Why? `...` is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like `arguments`.
 
     ```javascript
     // bad
@@ -1112,7 +1112,7 @@ Other Style Guides
 
   - [13.2](#13.2) <a name='13.2'></a> Use one `const` declaration per variable. eslint: [`one-var`](http://eslint.org/docs/rules/one-var.html) jscs: [`disallowMultipleVarDecl`](http://jscs.info/rule/disallowMultipleVarDecl)
 
-    > Why? It's easier to add new variable declarations this way, and you never have to worry about swapping out a `;` for a `,` or introducing punctuation-only diffs.
+    > Why? It's easier to add new variable declarations this way, and you never have to worry about swapping out a `;` for a `,` or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.
 
     ```javascript
     // bad
@@ -1339,6 +1339,7 @@ Other Style Guides
     ```
 
   - [15.4](#15.4) <a name='15.4'></a> For more information see [Truth Equality and JavaScript](http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll.
+
   - [15.5](#15.5) <a name='15.5'></a> Use braces to create blocks in `case` and `default` clauses that contain lexical declarations (e.g. `let`, `const`, `function`, and `class`).
 
   > Why? Lexical declarations are visible in the entire `switch` block but only get initialized when assigned, which only happens when its `case` is reached. This causes problems when multiple `case` clauses attempt to define the same thing.
@@ -2011,7 +2012,10 @@ Other Style Guides
     // => this.reviewScore = 9;
 
     // bad
-    const totalScore = this.reviewScore + '';
+    const totalScore = this.reviewScore + ''; // invokes this.reviewScore.valueOf()
+
+    // bad
+    const totalScore = this.reviewScore.toString(); // isn't guaranteed to return a string
 
     // good
     const totalScore = String(this.reviewScore);