docs: where clause limitations with feature request CTA. (#1773)

Partly addresses #1769
although it would be much better to add more technical details.


https://deploy-preview-1773--electric-next.netlify.app/docs/guides/shapes#where-clause

---------

Co-authored-by: Oleksii Sholik <[email protected]>

website/docs/guides/shapes.md

@@ -64,7 +64,22 @@ You can use logical operators like `AND` and `OR` to group multiple conditions,
 - `title='Electric' OR title='SQL'`
 - `title='Electric' AND status='todo'`
 
-Where clauses can only refer to columns in the target row; you can't perform joins or refer to other tables. Where clauses also can't use SQL functions like `count()`.
+> [!WARNING] Limitations
+> Electric needs to be able to evaluate where clauses outside of Postgres, so it only supports a limited set of SQL types and expressions right now.
+> 1. you can use columns of numerical types, `boolean`, `uuid`, `text`, `interval`, date and time types (with the exception of `timetz`)
+> 1. operators that work on those types: arithmetics, comparisons, boolean operators like `OR`, string operators like `LIKE`, etc.
+> 1. [Arrays](https://github.com/electric-sql/electric/issues/1767) and [Enums](https://github.com/electric-sql/electric/issues/1709) are not yet supported in where clauses
+>
+> For the full and up-to-date list of supported types, operators and functions, see their implementation in [`known_functions.ex`](https://github.com/electric-sql/electric/blob/main/packages/sync-service/lib/electric/replication/eval/env/known_functions.ex), while some expressions are handled in the [parser](https://github.com/electric-sql/electric/blob/main/packages/sync-service/lib/electric/replication/eval/parser.ex) (look for the `do_parse_and_validate_tree()` function).
+>
+> ---
+>
+> Some general rules that shape where clauses abide by are:
+> 1. where clauses can only refer to columns in the target row
+> 1. where clauses can't perform joins or refer to other tables
+> 1. where clauses can't use non-deterministic SQL functions like `count()` or `now()`
+>
+> If you need to use a data type or where clause feature that isn't yet supported, please feel free to [raise a Feature Request](https://github.com/electric-sql/electric/discussions/categories/feature-requests) on GitHub.
 
 ## Subscribing to shapes
 
@@ -180,4 +195,4 @@ When dropping a table from Postgres you need to *manually* delete all shapes tha
 This is especially important if you intend to recreate the table afterwards (possibly with a different schema) as the shape will contain stale data from the old table.
 Therefore, recreating the table only works if you first delete the shape.
 
-Electric does not yet automatically delete shapes when tables are dropped because Postgres does not stream DDL statements (such as `DROP TABLE`) on the logical replication stream that Electric uses to detect changes. However, we are actively exploring approaches for automated shape deletion in this [GitHub issue](https://github.com/electric-sql/electric/issues/1733).
+Electric does not yet automatically delete shapes when tables are dropped because Postgres does not stream DDL statements (such as `DROP TABLE`) on the logical replication stream that Electric uses to detect changes. However, we are actively exploring approaches for automated shape deletion in this [GitHub issue](https://github.com/electric-sql/electric/issues/1733).