docs(guide/filter): mention that filters can be used in directives

The doc mentions filters can be used in services and controllers but directives
aren't mentioned. This could lead to confusion for beginners.

Author: tamakisquare

docs/content/guide/filter.ngdoc

@@ -29,12 +29,12 @@ E.g. the markup `{{ 1234 | number:2 }}` formats the number 1234 with 2 decimal p
 {@link ng.filter:number `number`} filter. The resulting value is `1,234.00`.
 
 
-## Using filters in controllers and services
+## Using filters in controllers, services, and directives
 
-You can also use filters in controllers and services. For this, add a dependency with the name `<filterName>Filter`
-to your controller or service. E.g. using the dependency `numberFilter` will inject the number filter.
-The injected argument is a function that takes the value to format as first argument and filter parameters
-starting with the second argument.
+You can also use filters in controllers, services, and directives. For this, inject a dependency
+with the name `<filterName>Filter` to your controller/service/directive. E.g. using the dependency
+`numberFilter` will inject the number filter. The injected argument is a function that takes the
+value to format as first argument and filter parameters starting with the second argument.
 
 The example below uses the filter called {@link ng.filter:filter `filter`}.
 This filter reduces arrays into sub arrays based on