doc: explain how to use policies in migrations (#4141)

examples/rls/schema.sql

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+-- Enable row-level security on the users table.
+ALTER TABLE "users" ENABLE ROW LEVEL SECURITY;
+
+-- Create a policy that restricts access to rows in the users table based on the current tenant.
+CREATE POLICY tenant_isolation ON "users"
+    USING ("tenant_id" = current_setting('app.current_tenant')::integer);