multi: add the flag prune-revocation to perform the optional migration

sample-lnd.conf

@@ -1205,6 +1205,12 @@ litecoin.node=ltcd
 ; less RAM. Can only be used with a bolt database backend.
 ; db.no-graph-cache=true
 
+; Specify whether the optional migration for pruning old revocation logs
+; should be applied. This migration will only save disk space if there are open
+; channels prior to lnd@v0.15.0.
+; db.prune-revocation=false
+
+
 [etcd]
 
 ; Etcd database host.