docs: mention what behavior array-join-separator rule is trying to …

…avoid

Naturally, I think readers will be curious what the default behavior of `join` when this rule is not followed.

docs/rules/require-array-join-separator.md

@@ -1,6 +1,6 @@
 # Enforce using the separator argument with `Array#join()`
 
-It's better to make it clear what the separator is when calling [Array#join()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join).
+It's better to make it clear what the separator is when calling [Array#join()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join), instead of relying on the default comma (`,`) separator. 
 
 This rule is fixable.