add docstring to new exception

jupyter_server/services/sessions/sessionmanager.py

@@ -10,21 +10,22 @@
     # fallback on pysqlite2 if Python was build without sqlite
     from pysqlite2 import dbapi2 as sqlite3
 
+from dataclasses import dataclass, fields
+from typing import Union
+
 from tornado import web
 from traitlets import Instance, TraitError, Unicode, validate
 from traitlets.config.configurable import LoggingConfigurable
 
-from typing import Union
-
-from jupyter_server.utils import ensure_async
 from jupyter_server.traittypes import InstanceFromClasses
 from jupyter_server.utils import ensure_async
 
-from dataclasses import dataclass
-from dataclasses import fields
-
 
 class KernelSessionRecordConflict(Exception):
+    """Exception class to use when two KernelSessionRecords cannot
+    merge because of conflicting data.
+    """
+
     pass