document postgres-specific syntax

examples/master-detail-forms/insert_user.sql

@@ -1,5 +1,6 @@
 INSERT INTO user (id, first_name, last_name, email) VALUES (CAST($id AS INT), :"First name", :"Last name", :Email)
-ON CONFLICT (id) DO UPDATE SET first_name = excluded.first_name, last_name = excluded.last_name, email = excluded.email
+ON CONFLICT (id) DO -- this syntax is PostgreSQL-specific. In SQLite, use ON CONFLICT IGNORE.
+UPDATE SET first_name = excluded.first_name, last_name = excluded.last_name, email = excluded.email
 RETURNING
     'redirect' AS component,
     'edit_user.sql?id=' || id AS link; 

examples/user-authentication/create_user.sql

@@ -1,6 +1,6 @@
 INSERT INTO user_info (username, password_hash)
 VALUES (:username, sqlpage.hash_password(:password))
-ON CONFLICT (username) DO NOTHING
+ON CONFLICT (username) DO NOTHING -- this syntax is PostgreSQL-specific. In SQLite, use ON CONFLICT IGNORE.
 RETURNING 
     'redirect' AS component,
     'create_user_welcome_message.sql?username=' || :username AS link;