Document ViewSet.action (#5685)
Document ViewSet.action (#5685)

Closes #2941

Provides example of adjusting permission by action.
kgeorgy/django-rest-framework,tomchristie/django-rest-framework,kgeorgy/django-rest-framework,tomchristie/django-rest-framework,tomchristie/django-rest-framework,kgeorgy/django-rest-framework