Fix docs to clarify that update-where! doesn't call pre-update (or post-update)

[camsaul]

https://github.com/metabase/toucan/blob/master/docs/db-functions.md#update-where says update-where does not call pre-update while https://github.com/metabase/toucan/blob/master/docs/defining-models.md#pre-update says pre-update gets called, while post-update does not.