Clarify allowed cron values (#2425)

Author: treysp

docs/concepts/models/overview.md

@@ -169,7 +169,7 @@ Name is ***required*** and must be ***unique***.
 - End is used to determine the latest time needed to process the model. It can be an absolute date/time (`2022-01-01`), or a relative one (`1 year ago`).
 
 ### cron
-- Cron is used to schedule your model to process or refresh at a certain interval. It uses [croniter](https://github.com/kiorky/croniter) under the hood, so expressions such as `@daily` can be used.
+- Cron is used to schedule your model to process or refresh at a certain interval. It accepts a [cron expression](https://en.wikipedia.org/wiki/Cron) or any of `@hourly`, `@daily`, `@weekly`, or `@monthly`.
 
 ### interval_unit
 - Interval unit determines the granularity of data intervals for this model. By default the interval unit is automatically derived from the `cron` expression. Supported values are: `year`, `month`, `day`, `hour`, `half_hour`, `quarter_hour`, and `five_minute`.