[CLI] Valid list of services should present as hint for CLI backup services

Executed below command :

couchbase-cli backup-service -c 172.23.97.164:8091 -u Administrator -p password plan --add --name P6 --description descripdjsnfkashflJFASGNSALKFGNS --services searches ERROR: status - 400 ERROR: msg - invalid service searches

As in services passing parameter is predefined so it is good to be present as hint as whats values Services option can accept.

List is present in example section of docs.

https://docs.couchbase.com/server/7.0/cli/cbcli/couchbase-cli-backup-service-plan.html#examples