Pre-agg combination of config not sufficiently explained

[jlloyd-widen]

https://github.com/cube-js/cube/blob/cc4092e66d6963be7ceae6dafafd6bdedd2ee40b/docs/pages/reference/data-model/pre-aggregations.mdx#L961

In this section of the docs, it would helpful if it stated that you cannot use the incremental key when you are using the sql key.

For that matter, it would significantly help this particular page if some regular attention was made to what combinations of config can/should be used together and what cannot as it related, at least, to pre-aggs.

[igorlukanin]

@jlloyd-widen Great suggestion, thank you!