Docs: Elaborate on guard-for-in best practice (fixes #7071) (#7094)

Explains why a naive `foo.hasOwnProperty(key)` call is a bad idea in the `guard-for-in` page.

Hopefully prevents future users from being as confused as I was :) (or, maybe worse, ignoring the obscure `{}.hasOwnProperty.call(foo, key)` syntax)

docs/rules/guard-for-in.md

@@ -8,6 +8,8 @@ for (key in foo) {
 }
 ```
 
+Note that simply checking `foo.hasOwnProperty(key)` is likely to cause an error in some cases; see [no-prototype-builtins](no-prototype-builtins.md).
+
 ## Rule Details
 
 This rule is aimed at preventing unexpected behavior that could arise from using a `for in` loop without filtering the results in the loop. As such, it will warn when `for in` loops do not filter their results with an `if` statement.
@@ -34,6 +36,10 @@ for (key in foo) {
 }
 ```
 
+## Related Rules
+
+* [no-prototype-builtins](no-prototype-builtins.md)
+
 ## Further Reading
 
 * [Exploring JavaScript for-in loops](http://javascriptweblog.wordpress.com/2011/01/04/exploring-javascript-for-in-loops/)