docs(plugins) Try to clarify how ignore-plugin works (webpack#2130)

* Try to clarify how ignore-plugin works

The way `ignore-plugin` works wasn't very intuitive to me, so I'd like to try to clarify exactly what the regexes mean in case anyone else is similarly confused.

* Show broken code in 'diff' format

Author: jpage-godaddy

src/content/plugins/ignore-plugin.md

@@ -3,6 +3,7 @@ title: IgnorePlugin
 contributors:
   - simon04
   - byzyk
+  - DullReferenceException
 ---
 
 Prevent generation of modules for `import` or `require` calls matching the following regular expressions:
@@ -19,8 +20,24 @@ The following examples demonstrate a few ways this plugin can be used.
 
 ## Ignore Moment Locales
 
-As of [moment](https://momentjs.com/) 2.18, all locales are bundled together with the core library (see [this GitHub issue](https://github.com/moment/moment/issues/2373)). You can use the `IgnorePlugin` to stop any locale being bundled with moment:
+As of [moment](https://momentjs.com/) 2.18, all locales are bundled together with the core library (see [this GitHub issue](https://github.com/moment/moment/issues/2373)). 
+
+The `requestRegExp` parameter passed to `IgnorePlugin` is not tested against the resolved file names or absolute module names being imported or required, but rather against the _string_ passed to `require` or `import` _within the source code where the import is taking place_. For example, if you're trying to exclude `node_modules/moment/locale/*.js`, this won't work:
+
+```diff
+-new webpack.IgnorePlugin(/moment\/locale\//);
+```
+
+Rather, because `moment` imports with this code:
+
+```js
+require('./locale/' + name);
+```
+
+...your first regexp must match that `'./locale/'` string. The second `contextRegExp` parameter is then used to select specific directories from where the import took place. The following will cause those locale files to be ignored:
 
 ```js
 new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/);
 ```
+
+...which means "any require statement matching `'./locale'` from any directories ending with `'moment'` will be ignored.