Adde two "versionadded" markers, thanks to mYk for noticing.

docs/ref/models/fields.txt

@@ -1069,6 +1069,8 @@ define the details of how the relation works.
 
 .. attribute:: ForeignKey.db_constraint
 
+    .. versionadded:: 1.6
+
     Controls whether or not a constraint should be created in the database for
     this foreign key. The default is ``True``, and that's almost certainly what
     you want; setting this to ``False`` can be very bad for data integrity.
@@ -1229,6 +1231,8 @@ that control how the relationship functions.
 
 .. attribute:: ManyToManyField.db_constraint
 
+    .. versionadded:: 1.6
+
     Controls whether or not constraints should be created in the database for
     the foreign keys in the intermediary table. The default is ``True``, and
     that's almost certainly what you want; setting this to ``False`` can be