add some comments to the migration itself

taggit/migrations/0002_auto_20150616_2121.py

@@ -5,12 +5,18 @@ class Migration(migrations.Migration):
     dependencies = [("taggit", "0001_initial")]
 
     operations = [
+        # this migration was modified from previously being
+        # a ModifyIndexTogether operation.
+        #
+        # If you are a long-enough user of this library, the name
+        # of the index does not match what is written here. Please
+        # query the DB itself to find out what the name originally was.
         migrations.AddIndex(
             "taggeditem",
             models.Index(
                 fields=("content_type", "object_id"),
                 # this is not the name of the index in previous version,
-                # but this is ncessary to deal with index_together issues.
+                # but this is necessary to deal with index_together issues.
                 name="taggit_tagg_content_8fc721_idx",
             ),
         )