[3.0.x] Fixed #31368 -- Doc'd 'expression' parameter of Field.from_db_value()/Expression.convert_value().

Backport of 6929f13254 from master

docs/ref/models/expressions.txt

@@ -991,6 +991,8 @@ calling the appropriate methods on the wrapped expression.
         A hook allowing the expression to coerce ``value`` into a more
         appropriate type.
 
+        ``expression`` is the same as ``self``.
+
     .. method:: get_group_by_cols(alias=None)
 
         Responsible for returning the list of columns references by

docs/ref/models/fields.txt

@@ -2024,6 +2024,8 @@ Field API reference
         backend already returns the correct Python type, or the backend itself
         does the conversion.
 
+        ``expression`` is the same as ``self``.
+
         See :ref:`converting-values-to-python-objects` for usage.
 
         .. note::