Document multi-column join conditions in Sync Streams (#395)

* Document multi-column join conditions in Sync Streams

Generated-By: mintlify-agent

* Improve composite key example

---------

Co-authored-by: mintlify[bot] <109931778+mintlify[bot]@users.noreply.github.com>
Co-authored-by: Benita Volkmann <benita@journeyapps.com>

Author: mintlify[bot]

sync/supported-sql.mdx

@@ -270,6 +270,12 @@ Sync Streams support a subset of join syntax. The following rules define what is
 -- Valid: columns from one table
 SELECT comments.* FROM comments INNER JOIN issues ON comments.issue_id = issues.id
 
+-- Valid: multiple equality conditions (composite keys)
+SELECT comments.* FROM comments, issues
+  WHERE comments.issue_id = issues.id
+    AND comments.region = issues.region
+    AND issues.user_id = auth.user_id()
+
 -- Invalid: columns from multiple tables
 SELECT comments.*, issues.title FROM comments JOIN issues ON comments.issue_id = issues.id