docs: show how to install the release candidate instead of a canary

With `@next`, `ng add` and `ng update` currently install a canary release
(#3766). The quickstart and upgrade guide now follow each with
`npm install --save-exact @angular/fire@next`.

Author: armando-navarro

docs/install-and-setup.md

@@ -46,6 +46,12 @@ This installs AngularFire and configures your project. `ng add` will:
 1. Prompt you to select the features to enable and the Firebase project to use, signing you in to Firebase if needed.
 2. Add `provideFirebaseApp(...)`, along with a provider for each feature you select, to your app configuration (for example `app.config.ts`), with your Firebase configuration inlined. No environment files are created.
 
+`ng add` writes the version it installed into your `package.json`, and today that is a canary release such as `21.0.0-rc.1-canary.95b3de1` rather than the release candidate. To replace it with the release candidate, `21.0.0-rc.1`, run:
+
+```bash
+npm install --save-exact @angular/fire@next
+```
+
 ### 3. Inject `Firestore`
 
 Open `/src/app/app.ts` and make the following changes:

docs/version-21-upgrade.md

@@ -10,6 +10,12 @@ The release candidate is published under the `next` tag. From an app on Angular
 ng update @angular/core@21 @angular/cli@21 @angular/fire@next
 ```
 
+`ng update` writes the range `^21.0.0-rc.1` into your `package.json`. That range also matches canary releases, and npm currently installs one of them, such as `21.0.0-rc.1-canary.95b3de1`, instead of the release candidate. To install the release candidate itself, and stop npm from picking a canary release on a later install, change the entry to the exact version `21.0.0-rc.1` by running:
+
+```bash
+npm install --save-exact @angular/fire@next
+```
+
 The AngularFire update runs a migration that:
 
 - **Aligns your `firebase` dependency to `^12.18.0`.** AngularFire 21 requires Firebase JS SDK 12, at 12.18 or later. If your app requested an older `firebase`, whether that is 11 or an earlier 12, npm would install both that copy and the one AngularFire needs side by side, and the two copies reject each other's objects at runtime. The migration updates the dependency and reinstalls so you end up with a single copy. Verify with `npm ls firebase`.