Docs: list another related rule in no-undefined (#8467)

* Docs: add related rules, context to no-undefined

* Docs: tweak phrasing of background info for no-undefined rule

docs/rules/no-undefined.md

@@ -1,6 +1,6 @@
 # Disallow Use of `undefined` Variable (no-undefined)
 
-The `undefined` variable is unique in JavaScript because it is actually a property of the global object. As such, in ECMAScript 3 it was possible to overwrite the value of `undefined`. While ECMAScript 5 disallows overwriting `undefined`, it's still possible to shadow `undefined`, such as:
+The `undefined` variable in JavaScript is actually a property of the global object. As such, in ECMAScript 3 it was possible to overwrite the value of `undefined`. While ECMAScript 5 disallows overwriting `undefined`, it's still possible to shadow `undefined`, such as:
 
 ```js
 function doSomething(data) {
@@ -14,24 +14,15 @@ function doSomething(data) {
 }
 ```
 
-This represents a problem for `undefined` that doesn't exist for `null`, which is a keyword and primitive value that can neither be overwritten nor shadowed.
+Because `undefined` can be overwritten or shadowed, reading `undefined` can give an unexpected value. (This is not the case for `null`, which is a keyword that always produces the same value.) To guard against this, you can avoid all uses of `undefined`, which is what some style guides recommend and what this rule enforces. Those style guides then also recommend:
 
-All uninitialized variables automatically get the value of `undefined`:
-
-```js
-var foo;
-
-console.log(foo === undefined);     // true (assuming no shadowing)
-```
-
-For this reason, it's not necessary to explicitly initialize a variable to `undefined`.
-
-Taking all of this into account, some style guides forbid the use of `undefined`, recommending instead:
-
-* Variables that should be `undefined` are simply left uninitialized.
+* Variables that should be `undefined` are simply left uninitialized. (All uninitialized variables automatically get the value of `undefined` in JavaScript.)
 * Checking if a value is `undefined` should be done with `typeof`.
 * Using the `void` operator to generate the value of `undefined` if necessary.
 
+As an alternative, you can use the [no-global-assign](no-global-assign.md) and [no-shadow-restricted-names](no-shadow-restricted-names.md) rules to prevent `undefined` from being shadowed or assigned a different value. This ensures that `undefined` will always hold its original, expected value.
+
+
 ## Rule Details
 
 This rule aims to eliminate the use of `undefined`, and as such, generates a warning whenever it is used.
@@ -84,3 +75,5 @@ If you want to allow the use of `undefined` in your code, then you can safely tu
 
 * [no-undef-init](no-undef-init.md)
 * [no-void](no-void.md)
+* [no-shadow-restricted-names](no-shadow-restricted-names.md)
+* [no-global-assign](no-global-assign.md)