Fixed #25917 -- Clarified reversibility of RemoveField.

Thanks kaifeldhoff for the draft patch.

docs/ref/migration-operations.txt

@@ -157,9 +157,11 @@ RemoveField
 
 Removes a field from a model.
 
-Bear in mind that when reversed this is actually adding a field to a model;
-if the field is not nullable this may make this operation irreversible (apart
-from any data loss, which of course is irreversible).
+Bear in mind that when reversed, this is actually adding a field to a model.
+The operation is reversible (apart from any data loss, which of course is
+irreversible) if the field is nullable or if it has a default value that can be
+used to populate the recreated column. If the field is not nullable and does
+not have a default value, the operation is irreversible.
 
 AlterField
 ----------