Update documentation for cookie_secret (#1433)

Author: krassowski

jupyter_server/serverapp.py

@@ -1139,8 +1139,9 @@ def _default_cookie_secret_file(self) -> str:
         b"",
         config=True,
         help="""The random bytes used to secure cookies.
-        By default this is a new random number every time you start the server.
-        Set it to a value in a config file to enable logins to persist across server sessions.
+        By default this is generated on first start of the server and persisted across server
+        sessions by writing the cookie secret into the `cookie_secret_file` file.
+        When using an executable config file you can override this to be random at each server restart.
 
         Note: Cookie secrets should be kept private, do not share config files with
         cookie_secret stored in plaintext (you can read the value from a file).