Added NON_FIELD_ERRORS_KEY to serializer docs. Closes #1971.

docs/api-guide/settings.md

@@ -399,6 +399,12 @@ This should be a function with the following signature:
 
 Default: `'rest_framework.views.exception_handler'`
 
+#### NON_FIELD_ERRORS_KEY
+
+A string representing the key that should be used for serializer errors that do not refer to a specific field, but are instead general errors.
+
+Default: `'non_field_errors'`
+
 #### URL_FIELD_NAME
 
 A string representing the key that should be used for the URL fields generated by `HyperlinkedModelSerializer`.