Awareness of the meaning of Health in the overview panel by means of help hint

The overview panel shows `Health` with one moniker of {good, warning, critical}.

These monikers have an exact meaning defined by a comment in the code as:

```
// table health status:
// good : all shards (primary and replicated) are started
// warning : all primary shards are started && at least on replicated shard is not started
// critical : at least one primary shard is not started
```

It would be nice to have a help hint on hover over the moniker in the UI displaying the definition.
and/or
add a note in the overview document, defining the monikers.