improved docstring for BaseMigrationManager.get_migration_modules

Author: dantownsend

piccolo/apps/migrations/commands/base.py

@@ -30,7 +30,9 @@ def get_migration_modules(
         self, folder_path: str
     ) -> t.Dict[str, MigrationModule]:
         """
-        Import the migration modules and store them in a dictionary.
+        Imports the migration modules in the given folder path, and returns
+        a mapping of migration ID to the corresponding migration module.
+
         """
         migration_modules = {}