diff --git a/cylc/flow/job_runner_handlers/documentation.py b/cylc/flow/job_runner_handlers/documentation.py
index a6f6af983be..3b546ab4209 100644
--- a/cylc/flow/job_runner_handlers/documentation.py
+++ b/cylc/flow/job_runner_handlers/documentation.py
@@ -421,6 +421,14 @@ def submit(
         Returns:
             (ret_code, out, err)
 
+            ret_code:
+                Subprocess return code.
+            out:
+                Subprocess standard output, note this should be newline
+                terminated.
+            err:
+                Subprocess standard error.
+
         """
         raise NotImplementedError()