Raise error when `source=` use on a child.

Closes #3292

Add import to example for filtering

`from rest_framework import filters` was missing from one of the examples

Fix typos in comments

Cached the field's root and context property

Resolved issue with rendering of nested serializers form when value is None

Update 2-requests-and-responses.md

just a small error, the trailing slash in the url should be omitted when adding a .suffix

Raise error if passed a serializer instance

Add a method to return the string value used in `choices` of a `RelatedField`

The `display_value` method returns a text type.

Returns the text representation of the instance. Subclasses can override this method to provide a different display value used for populating the `choices` property.

Using the ModelSerializer with a model containing a GenericIPAddressField
produces duplicated validation error messages.