rabbit: add rabbit.mapping key to rabbit config

Problem: there is no documentation for the rabbit.mapping config key.

Add it.

Author: jameshcorbett

tutorials/lab/rabbit_config.rst

@@ -56,6 +56,10 @@ The ``rabbit`` config table captures site-general policies and options for
 Flux's interactions with the rabbits.
 
 
+**mapping** (string)
+  (required) Path to rabbitmapping file for the cluster, as generated by
+  `flux rabbitmapping`.
+
 **kubeconfig** (string)
   (optional) Path to kubeconfig file for Flux to use, ideally with restricted permissions.
   This can be left undefined if the file is placed at the path `~flux/.kube/config`