chore: add a comment to migration

Should be in 9172184 commit.

Relate to #1621

Author: php-coder

src/main/resources/liquibase/version/0.4.7/2023-07-27--collection_added_at.xml

@@ -12,6 +12,7 @@
 		</addColumn>
 
 		<!-- Unfortunately, H2 doesn't support UPDATE with JOIN -->
+		<!-- NOTE: In PostgreSQL it is illegal to prefix columns with table alias in a SET clause -->
 		<sql>
 			UPDATE collections_series cs
 			SET added_at = (