docs: document /index.html as importer during dev (#12911)

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>

docs/guide/api-plugin.md

@@ -159,7 +159,9 @@ The following hooks are called on each incoming module request:
 - [`load`](https://rollupjs.org/plugin-development/#load)
 - [`transform`](https://rollupjs.org/plugin-development/#transform)
 
-They also have an extended `options` parameter with additional Vite-specific properties. You can read more in the [SSR documentation](/guide/ssr#ssr-specific-plugin-logic).
+These hooks also have an extended `options` parameter with additional Vite-specific properties. You can read more in the [SSR documentation](/guide/ssr#ssr-specific-plugin-logic).
+
+Some `resolveId` calls' `importer` value may be an absolute path for a generic `index.html` at root as it's not always possible to derive the actual importer due to Vite's unbundled dev server pattern. For imports handled within Vite's resolve pipeline, the importer can be tracked during the import analysis phase, providing the correct `importer` value.
 
 The following hooks are called when the server is closed: