Extend the description of the config option `execution.target`

the current description for `execution.target` only lists `remote`, `local`, `yarn-per-job`, `yarn-session` and `kubernetes-session`. [1]

we should add `kubernetes-application` and `yarn-application` as well.

 [1] https://github.com/apache/flink/blob/master/flink-core/src/main/java/org/apache/flink/configuration/DeploymentOptions.java#L41-L46