Update documentation on host turndown

docs/reference.md

@@ -3318,6 +3318,19 @@ If you want more control over this process (Debian upgrades have been
 event-less for a while now, but it's not always been the case) you
 can of course run the upgrade manually.
 
+### Decommissioning a host
+
+When turning down a host, it is necessary, at some point, to
+reschedule the services that were there onto some other hosts. To
+achieve a smooth transition, this is best done while the host is still
+available.
+
+To do this, set the *turndown* attribute to *true* in the inventory
+for the host you want to turn down, and then run *float* once more.
+This should safely reschedule all services, and remove them from the
+target host. It is then possible to simply shut down the target host
+and wipe its data.
+
 # Example scenarios
 
 This section will look at some example scenarios and use cases for