Improve docs for include option in the adapter

docs/general/adapters.md

@@ -143,17 +143,22 @@ This adapter follows **version 1.0** of the [format specified](../jsonapi/schema
 
 ### Include option
 
-All built-in adapters allow to include the associated resources. Nested associated resources is also supported.
-Use `include` option for this:
+Which [serializer associations](https://github.com/rails-api/active_model_serializers/blob/master/docs/general/serializers.md#associations) are rendered can be specified using the `include` option. The option usage is consistent with [the include option in the JSON API spec](http://jsonapi.org/format/#fetching-includes), but is available in all adapters.
 
+Example of the usage:
 ```ruby
   render json: @posts, include: ['author', 'comments', 'comments.author']
   # or
   render json: @posts, include: 'author,comments,comments.author'
 ```
-**Note:** Serializer classes must declare associations explicitly, using `belongs_to` and `has_many` and etc., for `include` option to work.
 
-For JSON API adapter associated resources will be gathered in the `"included"` member.
+The format of the `include` option can be either:
+
+- a String composed of a comma-separated list of [relationship paths](http://jsonapi.org/format/#fetching-includes).
+- an Array of Symbols and Hashes.
+- a mix of both.
+
+An empty string or an empty array will prevent rendering of any associations.
 
 In addition, two types of wildcards may be used:
 
@@ -166,11 +171,6 @@ These can be combined with other paths.
   render json: @posts, include: '**' # or '*' for a single layer
 ```
 
-The format of the `include` option can be either:
-
-- a String composed of a comma-separated list of [relationship paths](http://jsonapi.org/format/#fetching-includes).
-- an Array of Symbols and Hashes.
-- a mix of both.
 
 The following would render posts and include:
 
@@ -184,6 +184,20 @@ It could be combined, like above, with other paths in any combination desired.
   render json: @posts, include: 'author.comments.**'
 ```
 
+**Note:** Wildcards are AMS-specific, they are not part of the JSON API spec.
+
+The default include for the JSON API adapter is no associations. The default for the JSON and Attributes adapters is all associations.
+
+For the JSON API adapter associated resources will be gathered in the `"included"` member. For the JSON and Attributes
+adapters associated resources will be rendered among the other attributes.
+
+Only for the JSON API adapter you can specify, which attributes of associated resources will be rendered. This feature
+is called [sparse fieldset](http://jsonapi.org/format/#fetching-sparse-fieldsets):
+
+```ruby
+  render json: @posts, include: 'comments', fields: { comments: ['content', 'created_at'] }
+```
+
 ##### Security Considerations
 
 Since the included options may come from the query params (i.e. user-controller):