Merge pull request pallets-eco#761 from matanw/comment_init_app

add comment about db.init_app

Author: jab

flask_sqlalchemy/__init__.py

@@ -810,6 +810,9 @@ def init_app(self, app):
         of an application not initialized that way or connections will
         leak.
         """
+        # We intentionally don't set self.app = app, to support multiple
+        # applications. If the app is passed in the constructor,
+        # we set it and don't support multiple applications.
         if (
             'SQLALCHEMY_DATABASE_URI' not in app.config and
             'SQLALCHEMY_BINDS' not in app.config