docs(errors/$injector/nomod): add info about forgetting to load the module file

Closes #3752

docs/content/error/$injector/nomod.ngdoc

@@ -3,24 +3,31 @@
 @fullName Module Unavailable
 @description
 
-This error occurs when trying to "re-open" a module that has not yet been defined.
+This error occurs when you declare a dependency on a module that isn't defined anywhere or hasn't
+been loaded in the current browser context.
+
+When you receive this error, check that the name of the module in question is correct and that the
+file in which this module is defined has been loaded (either via `<script>` tag, loader like
+require.js, or testing harness like karma).
+
+A less common reason for this error is trying to "re-open" a module that has not yet been defined.
 
 To define a new module, call {@link angular.module angular.module} with a name
 and an array of dependent modules, like so:
 
-```
+```js
 // When defining a module with no module dependencies,
-// the requires array should be defined and empty.
+// the array of dependencies should be defined and empty.
 var myApp = angular.module('myApp', []);
 ```
 
 To retrieve a reference to the same module for further configuration, call
-`angular.module` without the `requires` array.
+`angular.module` without the array argument.
 
-```
+```js
 var myApp = angular.module('myApp');
 ```
 
-Calling `angular.module` without the `requires` array when the module has not yet
-been defined causes this error to be thrown. To fix it, define your module with
-a name and an empty array, as in the first example above.
+Calling `angular.module` without the array of dependencies when the module has not yet been defined
+causes this error to be thrown. To fix it, define your module with a name and an empty array, as in
+the first example above.