[2.2.x] Fixed #30670 -- Doc'd SchemaEditor.add/remove_constraint().

Backport of af08a54415 from master

AUTHORS

@@ -27,6 +27,7 @@ answer newbie questions, and generally made Django that much better:
     Ahmad Al-Ibrahim
     Ahmed Eltawela <https://github.com/ahmedabt>
     ajs <adi@sieker.info>
+    Akash Agrawal <akashrocksha@gmail.com>
     Akis Kesoglou <akiskesoglou@gmail.com>
     Aksel Ethem <aksel.ethem@gmail.com>
     Akshesh Doshi <aksheshdoshi+django@gmail.com>

docs/ref/schema-editor.txt

@@ -81,6 +81,24 @@ Adds ``index`` to ``model``’s table.
 
 Removes ``index`` from ``model``’s table.
 
+``add_constraint()``
+--------------------
+
+.. method:: BaseDatabaseSchemaEditor.add_constraint(model, constraint)
+
+.. versionadded:: 2.2
+
+Adds ``constraint`` to ``model``'s table.
+
+``remove_constraint()``
+-----------------------
+
+.. method:: BaseDatabaseSchemaEditor.remove_constraint(model, constraint)
+
+.. versionadded:: 2.2
+
+Removes ``constraint`` from ``model``'s table.
+
 ``alter_unique_together()``
 ---------------------------