docs(directive): link to directive registration api function

Originally, this issue was regarding documenting `restrict: 'CM'` in the directive guide, but it
was pointed out that the restrict documentation is covered in the $compile documentation. Because of
this, a link was simply added to the $compile documentation.

However, the wording suggests that it's actually linking to the directive registration function, in
$compileProvider, so the docs will link there instead. There is a link only a paragraph below to the
$compile documentation, so this does not hurt.

Closes #5516

docs/content/guide/directive.ngdoc

@@ -174,9 +174,9 @@ For example, we could fix the example above by instead writing:
 
 ## Creating Directives
 
-First let's talk about the API for registering directives. Much like controllers, directives are
-registered on modules. To register a directive, you use the `module.directive` API.
-`module.directive` takes the
+First let's talk about the {@link api/ng.$compileProvider#methods_directive API for registering directives}. Much like
+controllers, directives are registered on modules. To register a directive, you use the
+`module.directive` API. `module.directive` takes the
 {@link guide/directive#creating-custom-directives_matching-directives normalized} directive name
 followed by a **factory function.** This factory function should return an object with the different
 options to tell `$compile` how the directive should behave when matched.