diff --git a/cylc/flow/job_runner_handlers/documentation.py b/cylc/flow/job_runner_handlers/documentation.py
index 735b1801b16..1188e0e052c 100644
--- a/cylc/flow/job_runner_handlers/documentation.py
+++ b/cylc/flow/job_runner_handlers/documentation.py
@@ -419,6 +419,14 @@ 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()