Add comments for parallel StatefulSet update #175

Right now, the searcher and indexer StatefulSets are deployed one pod at a time, which can take a long time when users run a large number of pods.

StatefulSets have a feature called .spec.updateStrategy.rollingUpdate.maxUnavailable, which allows pods to be updated in parallel. However, this feature is currently disabled by default. Once it is enabled by default, users should be able to set this field and deploy those pods in parallel.

Note that in order to use this feature, podManagementPolicy needs to be set to Parallel instead of OrderedReady, which is the default. However, since this field is immutable, we cannot change the default policy for existing StatefulSets. The best we can probably do is add comments in values.yaml.