docs: say what a stopped ng add leaves behind

Author: armando-navarro

docs/install-and-setup.md

@@ -6,7 +6,7 @@
   - Setup uses the Firebase CLI (`firebase-tools`). Install it with `npm install -g firebase-tools`, then run `firebase login`. If `ng add` can't find a copy, it installs the newest version for you.
   - Run `ng add` yourself, not through an AI agent.
     - In AngularFire 21.0.0-rc.1 and earlier, `ng add` run by an agent with firebase-tools 15.26 or later stops after you choose features, with `Cannot read properties of undefined (reading 'email')`.
-    - If it already stopped, run `ng add @angular/fire@next` again yourself. The stopped run leaves nothing to clean up.
+    - If it already stopped, run `ng add @angular/fire@next` again yourself. The stopped run leaves only an empty `firebase.json`, which the second run reuses.
   - A copy older than 14 stops setup with `firebase-tools version 14.0.0+ is required, please upgrade and run again`, and `ng add` does not upgrade a copy you already have.
 - **Harmless CLI noise.** The Firebase CLI may print a `punycode` deprecation warning or ask about enabling extra features (for example Gemini) during setup. These come from the CLI, not from AngularFire, and are safe to ignore.