doc: add comment on method to set savepoint_limit dynamically

savepoint_limit isn't a real config option as in config.ini. I
generate one on the fly for use in the backend during import.

While this is arguably a valid config option, it is also something
that is niche (import a tracker to postgres) and adds clutter to
config.ini.

Author: rouilj

roundup/admin.py

@@ -1058,6 +1058,9 @@ def do_import(self, args, import_files=True):
         # default value is 10000, only go through this if default
         # is different.
         if self.settings['savepoint_limit'] != 10000:
+            # create a new option on the fly in the config under the
+            # rdbms section. It is used by the postgresql backend's
+            # checkpoint_data method.
             self.db.config.add_option(Option(self.db.config,
                                              "rdbms", "savepoint_limit"))
             self.db.config.options["RDBMS_SAVEPOINT_LIMIT"].set(