Correct notes on BQ constraint definition and enforcement. (#4679)

## What are you changing in this pull request and why?

Correct notes on BQ constraint definition and enforcement. Most notes
are correct but this section had been missed.

## Checklist

- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
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review how to [version a whole
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and [version a block of
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- [ ] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

Adding new pages (delete if not applicable):
- [ ] Add page to `website/sidebars.js`
- [ ] Provide a unique filename for the new page

Removing or renaming existing pages (delete if not applicable):
- [ ] Remove page from `website/sidebars.js`
- [ ] Add an entry `website/static/_redirects`
- [ ] Run link testing locally with `npm run build` to update the links
that point to the deleted page

---------

Co-authored-by: mirnawong1 <89008547+mirnawong1@users.noreply.github.com>
Co-authored-by: Leona B. Campbell <3880403+runleonarun@users.noreply.github.com>

website/docs/reference/resource-properties/constraints.md

@@ -300,7 +300,7 @@ select
 
 <div warehouse="BigQuery">
 
-BigQuery allows defining `not null` constraints. However, it does _not_ support or enforce the definition of unenforced constraints, such as `primary key`.
+BigQuery allows defining and enforcing `not null` constraints, and defining (but _not_ enforcing) `primary key` and `foreign key` constraints (which can be used for query optimization). BigQuery does not support defining or enforcing other constraints. For more information, refer to [Platform constraint support](/docs/collaborate/govern/model-contracts#platform-constraint-support)
 
 Documentation: https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language