doc: improve description of module exports

- Do not use word alias, it isn't well defined
- Fix return value of require() example, which confusingly was not the
  exported API as it should have been.
- Fix the require() example, which claimed that the module exported `0`,
  when it exports `some_func`.
- Describe best practice in keeping exports and module.exports bound
  together.
- Describe why exports exists
- Remove reference to magic, which is also not well defined

PR-URL: nodejs#9622
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: sam-github

doc/api/modules.md

@@ -529,34 +529,51 @@ const x = require('./x');
 console.log(x.a);
 ```
 
-#### exports alias
+#### exports shortcut
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-The `exports` variable that is available within a module starts as a reference
-to `module.exports`. As with any variable, if you assign a new value to it, it
-is no longer bound to the previous value.
+The `exports` variable is available within a module's file-level scope, and is
+assigned the value of `module.exports` before the module is evaluated.
+
+It allows a shortcut, so that `module.exports.f = ...` can be written more
+succinctly as `exports.f = ...`. However, be aware that like any variable, if a
+new value is assigned to `exports`, it is no longer bound to `module.exports`:
+
+```js
+module.exports.hello = true; // Exported from require of module
+exports = { hello: false };  // Not exported, only available in the module
+```
+
+When the `module.exports` property is being completely replaced by a new
+object, it is common to also reassign `exports`, for example:
+
+```js
+module.exports = exports = function Constructor() {
+    // ... etc.
+```
 
 To illustrate the behavior, imagine this hypothetical implementation of
-`require()`:
+`require()`, which is quite similar to what is actually done by `require()`:
 
 ```js
 function require(...) {
-  // ...
+  var module = { exports: {} };
   ((module, exports) => {
-    // Your module code here
-    exports = some_func;        // re-assigns exports, exports is no longer
-                                // a shortcut, and nothing is exported.
-    module.exports = some_func; // makes your module export 0
+    // Your module code here. In this example, define a function.
+    function some_func() {};
+    exports = some_func;
+    // At this point, exports is no longer a shortcut to module.exports, and
+    // this module will still export an empty default object.
+    module.exports = some_func;
+    // At this point, the module will now export some_func, instead of the
+    // default object.
   })(module, module.exports);
-  return module;
+  return module.exports;
 }
 ```
 
-As a guideline, if the relationship between `exports` and `module.exports`
-seems like magic to you, ignore `exports` and only use `module.exports`.
-
 ### module.filename
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