doc: explain how to use enum types properly (#4137)

examples/enumtypes/atlas.hcl

@@ -0,0 +1,15 @@
+data "composite_schema" "app" {
+  # Load first custom types first.
+  schema "public" {
+    url = "file://schema.sql"
+  }
+  # Second, load the Ent schema.
+  schema "public" {
+    url = "ent://ent/schema"
+  }
+}
+
+env "local" {
+  src = data.composite_schema.app.url
+  dev = "docker://postgres/15/dev?search_path=public"
+}