More helpful cylc insert deprecation message (#6495)

Author: wxtim

cylc/flow/scripts/cylc.py

@@ -283,7 +283,8 @@ def get_version(long=False):
     'gscan':
         'cylc gscan has been removed, use the web UI',
     'insert':
-        'inserting tasks is now done automatically',
+        'Insertion is no longer required, `cylc set` and `cylc trigger`'
+        ' will insert tasks automatically.',
     'jobscript':
         'cylc jobscript has been removed',
     'nudge':