Update some comments to match a renamed method

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/migration.py

@@ -86,7 +86,8 @@ class MigrationManager(object):
         InstallabilityTester.
 
 
-        Unlike doop_source, this will not modify any data structure.
+        Unlike migrate_item_to_target_suite, this will not modify
+        any data structure.
         """
         # local copies for better performances
         source_name = item.package

britney2/transaction.py

@@ -28,7 +28,7 @@ class MigrationTransactionState(object):
 
     @property
     def undo_items(self):
-        """Only needed by a doop_source for the "hint"-hint case"""
+        """Only needed by a _apply_item_to_target_suite for the "hint"-hint case"""
         yield from self._undo_items
 
     def commit(self):