Add a comment about not importing defineExpose (vuejs#2140)

Author: skirtles-code

src/guide/essentials/template-refs.md

@@ -227,6 +227,7 @@ import { ref } from 'vue'
 const a = 1
 const b = ref(2)
 
+// Compiler macros, such as defineExpose, don't need to be imported
 defineExpose({
   a,
   b