Add warnings about global adapter config to docs (#2176)

Author: mrpinsky

CHANGELOG.md

@@ -20,6 +20,8 @@ Fixes:
 
 Misc:
 
+- #2176 Documentation for global adapter config (@mrpinsky)
+
 ### [v0.10.6 (2017-05-01)](https://github.com/rails-api/active_model_serializers/compare/v0.10.5...v0.10.6)
 
 Fixes:

docs/general/adapters.md

@@ -36,6 +36,12 @@ The `Attributes` adapter does not include a root key. It is just the serialized
 
 Use either the `JSON` or `JSON API` adapters if you want the response document to have a root key.
 
+***IMPORTANT***: Adapter configuration has *no effect* on a serializer instance
+being used directly. That is, `UserSerializer.new(user).as_json` will *always*
+behave as if the adapter were the 'Attributes' adapter. See [Outside Controller
+Usage](../howto/outside_controller_use.md) for more details on recommended
+usage.
+
 ## Built in Adapters
 
 ### Attributes - Default

docs/howto/add_root_key.md

@@ -18,6 +18,13 @@ In order to add the root key you need to use the ```JSON``` Adapter, you can cha
 ActiveModelSerializers.config.adapter = :json
 ```
 
+Note that adapter configuration has no effect on a serializer that is called
+directly, e.g. in a serializer unit test. Instead, something like
+`UserSerializer.new(user).as_json` will *always* behave as if the adapter were
+the 'Attributes' adapter. See [Outside Controller
+Usage](../howto/outside_controller_use.md) for more details on recommended
+usage.
+
 You can also specify a class as adapter, as long as it complies with the ActiveModelSerializers adapters interface.
 It will add the root key to all your serialized endpoints.