docs(tables): document reference columns

Author: j15z

apps/docs/content/docs/tables/index.mdx

@@ -27,8 +27,9 @@ Every column has a type, which decides how its values are stored and validated.
 | **Expiration** | An absolute row expiration time, stored as Unix epoch seconds (seconds since January 1, 1970 UTC) | `1773671400` |
 | **JSON** | An object or array | `{ "tier": "pro" }` |
 | **Select** | One of a fixed set of options, or several | `Pro` |
+| **Reference** | A row ID from another table in your workspace | `row_123` |
 
-Types are enforced as you enter values, so a Number column only takes numbers.
+Types are enforced as you enter values, so a Number column only takes numbers. A Reference column is intentionally different for now: it stores the row ID as plain text without checking that the row exists in the selected table.
 
 A Currency column stores a plain number and renders it in the currency you choose for that column, so filters, sorts, and exports all see the amount itself. Changing a column's currency relabels it — it does not convert the amounts.