[docs] document the schema of key concepts such as "filter"

The `setFilterUpdater` here https://github.com/keplergl/kepler.gl/blob/master/docs/api-reference/reducers/vis-state.md#setfilterupdater says:
```
- `action` **[Object][63]** action
- `action.idx` **[Number][67]** `idx` of filter to be updated
- `action.prop` **[string][64]** `prop` of filter, e,g, `dataId`, `name`, `value`
```
However the schema of "filter" is found in this documentation. Since filter is a common concept, would be nice to document it.