docs(config): improve node documentation (#1368)

The documentation about using built-in Node.js modules was very poor
(i.e. non-existent). This expands the documentation of the "node"
configuration option, and shows how one can require built-in modules if
desired.

Furthermore, all possible effects of the options are explicitly
documented. Instead of being vague of what happens when `false` is used,
it is explicitly spelled out what happens.

References:

-
https://github.com/webpack/webpack/blob/a589a6c9789a9d342fc630e36ab81827dd20289b/lib/WebpackOptionsApply.js
  shows when the NodeStuffPlugin and NodeSourcePlugin plugins are used.
- https://github.com/webpack/webpack/blob/a589a6c9789a9d342fc630e36ab81827dd20289b/lib/NodeStuffPlugin.js
  is the plugin that is used for every target.
- https://github.com/webpack/webpack/blob/a589a6c9789a9d342fc630e36ab81827dd20289b/lib/node/NodeSourcePlugin.js
  is the plugin that is only used for "web" and "webworker" targets.
- https://github.com/webpack/webpack/blob/a589a6c9789a9d342fc630e36ab81827dd20289b/lib/MultiModule.js#L65-L67
  is the generated "webpackMissingModule" code for unresolved modules.
  (when the NodeSourcePlugin is not used, the environment's "require" is
  used. In Node.js, this also throws a "Cannot find module 'modulename'"
  error (with single quotes instead of double quotes)).

Author: Rob--W

content/configuration/node.md

@@ -5,9 +5,12 @@ contributors:
   - sokra
   - skipjack
   - oneforwonder
+  - Rob--W
 ---
 
-These options configure whether to polyfill or mock certain [Node.js globals](https://nodejs.org/docs/latest/api/globals.html) and modules. This allows code originally written for the Node.js environment to run in other environments like the browser. This feature is provided by webpack's internal [`NodeStuffPlugin`](https://github.com/webpack/webpack/blob/master/lib/NodeStuffPlugin.js).
+These options configure whether to polyfill or mock certain [Node.js globals](https://nodejs.org/docs/latest/api/globals.html) and modules. This allows code originally written for the Node.js environment to run in other environments like the browser.
+
+This feature is provided by webpack's internal [`NodeStuffPlugin`](https://github.com/webpack/webpack/blob/master/lib/NodeStuffPlugin.js) plugin. If the target is "web" (default) or "webworker", the [`NodeSourcePlugin`](https://github.com/webpack/webpack/blob/master/lib/node/NodeSourcePlugin.js) plugin is also activated.
 
 
 ## `node`
@@ -19,7 +22,7 @@ This is an object where each property is the name of a Node global or module and
 - `true`: Provide a polyfill.
 - `"mock"`: Provide a mock that implements the expected interface but has little or no functionality.
 - `"empty"`: Provide an empty object.
-- `false`: Provide nothing. Code that expects this object to be defined may crash.
+- `false`: Provide nothing. Code that expects this object may crash with a `ReferenceError`. Code that attempts to import the module using `require('modulename')` may trigger a `Cannot find module "modulename"` error.
 
 W> Not every Node global supports all four options. The compiler will throw an error for property-value combinations that aren't supported (e.g. `process: 'empty'`). See the sections below for more details.
 
@@ -34,10 +37,12 @@ node: {
   __dirname: "mock",
   Buffer: true,
   setImmediate: true
+
+  // See "Other node core libraries" for additional options.
 }
 ```
 
-Since webpack 3.0.0, the `node` option may be set to `false` to turn off the `NodeSourcePlugin` completely.
+Since webpack 3.0.0, the `node` option may be set to `false` to completely turn off the `NodeStuffPlugin` and `NodeSourcePlugin` plugins.
 
 
 ## `node.console`
@@ -109,9 +114,13 @@ Default: `true`
 
 `boolean | "mock" | "empty"`
 
-Many other Node.js core libraries can be configured as well. See the list of [Node.js core libraries and their polyfills](https://github.com/webpack/node-libs-browser).
+W> This option is only activated (via `NodeSourcePlugin`) when the target is unspecified, "web" or "webworker".
+
+Polyfills for Node.js core libraries from [`node-libs-browser`](https://github.com/webpack/node-libs-browser) are used if available, when the `NodeSourcePlugin` plugin is enabled. See the list of [Node.js core libraries and their polyfills](https://github.com/webpack/node-libs-browser#readme).
+
+By default, webpack will polyfill each library if there is a known polyfill or do nothing if there is not one. In the latter case, webpack will behave as if the module name was configured with the `false` value.
 
-By default, Webpack will polyfill each library if there is a known polyfill or do nothing if there is not one.
+T> To import a built-in module, use [`__non_webpack_require__`](/api/module-variables/#__non_webpack_require__-webpack-specific-), i.e. `__non_webpack_require__('modulename')` instead of `require('modulename')`.
 
 Example: