docs(auth): provide auth DI tokens documentation (#2809)

Author: KingDarBoja

docs/auth/getting-started.md

@@ -38,6 +38,118 @@ export class AppComponent {
 }
 ```
 
+## Configuration with Dependency Injection
+
+The AngularFireAuth Module provides several DI tokens to further configure your
+authentication process.
+
+### Configure 
+
+Using the `SETTINGS` DI Token (*default: null*), we can set the current Auth
+instance's settings. This is used to edit/read configuration related options
+like app verification mode for phone authentication, which is useful for
+[testing](https://cloud.google.com/identity-platform/docs/test-phone-numbers).
+
+```ts
+import { SETTINGS as AUTH_SETTINGS } from '@angular/fire/auth';
+
+@NgModule({
+  // ... Existing configuration
+  providers: [
+    // ... Existing Providers
+    { provide: AUTH_SETTINGS, useValue: { appVerificationDisabledForTesting: true } },
+  ]
+})
+export class AppModule { }
+```
+
+Read more at [Firebase Auth Settings](https://firebase.google.com/docs/reference/js/firebase.auth.AuthSettings).
+
+### Use Current Browser Language
+
+Using the `USE_DEVICE_LANGUAGE` DI Token (*default: null*), which is a boolean
+that allow you to set the current language to the default device/browser
+preference. This allows to localize emails but be aware that this only applies
+if you use the standard template provided by Firebase. 
+
+```ts
+import { USE_DEVICE_LANGUAGE } from '@angular/fire/auth';
+
+@NgModule({
+  // ... Existing configuration
+  providers: [
+    // ... Existing Providers
+    { provide: USE_DEVICE_LANGUAGE, useValue: true },
+  ]
+})
+export class AppModule { }
+```
+
+If you want to set a different language, you can use `LANGUAGE_CODE` DI Token
+(*default: null*).
+
+More info at the [firebase auth docs](https://firebase.google.com/docs/reference/js/firebase.auth.Auth#languagecode).
+
+```ts
+import { LANGUAGE_CODE } from '@angular/fire/auth';
+
+@NgModule({
+  // ... Existing configuration
+  providers: [
+    // ... Existing Providers
+    { provide: LANGUAGE_CODE, useValue: 'fr' },
+  ]
+})
+export class AppModule { }
+```
+
+### Persistence
+
+Firebase Auth default behavior is to persist a user's session even after the
+user closes the browser. To change the current type of persistence on the
+current Auth instance for the currently saved Auth session and apply this type
+of persistence for future sign-in requests, including sign-in with redirect
+requests, you can use the `PERSISTENCE` DI Token (*default: null*).
+
+The possible types are `'local'`, `'session'` or `'none'`. Read more at 
+[authentication state persistence](https://firebase.google.com/docs/auth/web/auth-state-persistence),
+
+```ts
+import { PERSISTENCE } from '@angular/fire/auth';
+
+@NgModule({
+  // ... Existing configuration
+  providers: [
+    // ... Existing Providers
+    { provide: PERSISTENCE, useValue: 'session' },
+  ]
+})
+export class AppModule { }
+```
+
+### Tenant
+
+If you need to use multi-tenancy, you can set the current Auth instance's tenant
+ID using `TENANT_ID` DI Token (*default: null*).
+
+More tutorials regarding this topic are _coming soon_.
+
+```ts
+import { TENANT_ID } from '@angular/fire/auth';
+
+@NgModule({
+  // ... Existing configuration
+  providers: [
+    // ... Existing Providers
+    { provide: TENANT_ID, useValue: 'tenant-id-app-one' },
+  ]
+})
+export class AppModule { }
+```
+
+- [Multi-Tenancy Authentication](https://cloud.google.com/identity-platform/docs/multi-tenancy-authentication)
+- [Firebase Auth Tenant](https://firebase.google.com/docs/reference/js/firebase.auth.Auth#tenantid)
+
 ## UI Libraries
 
 - Material Design : [ngx-auth-firebaseui](https://github.com/AnthonyNahas/ngx-auth-firebaseui)