Scan CLI warning message. [skip ci]

cylc/flow/scripts/scan.py

@@ -60,6 +60,7 @@
 import asyncio
 import json
 from pathlib import Path
+from sys import stderr
 from typing import Callable, Optional, TYPE_CHECKING
 
 from ansimarkup import ansiprint as cprint
@@ -233,19 +234,17 @@ def get_option_parser() -> COP:
         '--colour-blind', '--color-blind',
         help=(
             "Don't depend on colour to convey information. "
-            ' Use this rather than --color=never so you still get bold text.'
+            'Use this rather than --color=never so you still get bold text.'
         ),
         action='store_true'
     )
 
     parser.add_option(
         '--ping',
         help=(
-            'Test the connection to the flow. Scan normally just reads flow'
-            ' contact files, but --ping forces a connection to the scheduler'
-            ' and removes the contact file if it is not found to be running'
-            " (this can happen if the scheduler gets killed and can't clean"
-            ' up after itself).'
+            "Connect to schedulers and remove contact files if they are not "
+            "found to be running. Contact files can be left behind when "
+            "schedulers get killed."
         ),
         action='store_true'
     )
@@ -578,6 +577,14 @@ async def main(
 
     await scanner(opts, write, scan_dir)
 
+    if opts.format != 'rich' and not opts.ping:
+        print(
+            "By default 'cylc scan' just reads workflow contact files, which"
+            "\ncan be left behind if a scheduler gets killed. Use '--ping'"
+            "\nto attempt connection and remove bad contact files.",
+            file=stderr
+        )
+
 
 @cli_function(get_option_parser)
 def cli(_, opts: 'Values', color: bool) -> None: