Removed hint from fields.E310 message in system check docs.
This is the only documented hint.
@@ -219,8 +219,7 @@ Related fields
* **fields.E309**: Reverse query name ``<related query name>`` must not contain
``'__'``.
* **fields.E310**: No subset of the fields ``<field1>``, ``<field2>``, ... on
- model ``<model>`` is unique. Add ``unique=True`` on any of those fields or
- add at least a subset of them to a unique_together constraint.
+ model ``<model>`` is unique.
* **fields.E311**: ``<model>`` must set ``unique=True`` because it is
referenced by a ``ForeignKey``.
* **fields.E312**: The ``to_field`` ``<field name>`` doesn't exist on the
