docs: use enforce post for CSS metadata (#22467)

Author: Sean-Kenneth-Doherty

docs/guide/api-plugin.md

@@ -526,6 +526,7 @@ Example:
 function outputMetadataPlugin(): Plugin {
   return {
     name: 'output-metadata-plugin',
+    enforce: 'post',
     generateBundle(_, bundle) {
       for (const output of Object.values(bundle)) {
         const css = output.viteMetadata?.importedCss