Amend ViewSet docs to warn of potential problem

Fix missing validated_data in `raise_errors_on_nested_writes` (#2221)

I went through this exact problem and so thought the docs might
benefit from a small hint at the appropriate point.

Update the tutorial against DRF v3 (part 4 & 5)

PrimaryKeyRelatedField now needs a queryset argument.
urls now don't use urlpatterns.

Raise error if `fields` on serializer is not a list of strings.

Better serializer errors for nested writes.

Clarify read-only & `unique_together` Documentation

Clean up compat code

Escape \u2028 and \u2029 in JSON output.

Bugfix/tutorial 1

Merge upstream

Added missing default style for FileField

Renamed validated_attrs to validated_data to be more in line with other code