Fixed #29641 -- Added support for unique constraints in Meta.constraints.

This constraint is similar to Meta.unique_together but also allows
specifying a name.

Co-authored-by: Ian Foote <python@ian.feete.org>

docs/ref/models/constraints.txt

@@ -43,3 +43,28 @@ ensures the age field is never less than 18.
 .. attribute:: CheckConstraint.name
 
 The name of the constraint.
+
+``UniqueConstraint``
+====================
+
+.. class:: UniqueConstraint(*, fields, name)
+
+    Creates a unique constraint in the database.
+
+``fields``
+----------
+
+.. attribute:: UniqueConstraint.fields
+
+A list of field names that specifies the unique set of columns you want the
+constraint to enforce.
+
+For example ``UniqueConstraint(fields=['room', 'date'], name='unique_location')``
+ensures only one location can exist for each ``date``.
+
+``name``
+--------
+
+.. attribute:: UniqueConstraint.name
+
+The name of the constraint.