docs: Add AZURE_AD_ALLOWED_PRINCIPALS doc

Author: yaegashi

docs/5-add-identity.md

@@ -83,6 +83,7 @@ We'll create two GitHub apps: one for testing locally and another for production
 AZURE_AD_CLIENT_ID=
 AZURE_AD_CLIENT_SECRET=
 AZURE_AD_TENANT_ID=
+AZURE_AD_ALLOWED_PRINCIPALS=
 ```
 
 ## Configure an admin user

docs/7-environment-variables.md

@@ -17,6 +17,7 @@ Below are the required environment variables, to be added to the Azure Portal or
 | `AZURE_AD_CLIENT_ID`                                                                      |                                                | The client id specific to the application                                                                                                 |
 | `AZURE_AD_CLIENT_SECRET`                                                                  |                                                | The client secret specific to the application                                                                                             |
 | `AZURE_AD_TENANT_ID`                                                                      |                                                | The organisation Tenant ID                                                                                                                |
+| `AZURE_AD_ALLOWED_PRINCIPALS`                                                             |                                                | Comma separated list of Object IDs for users/groups that are allowed to log in. **All authenticated users are allowed by default**        |
 | `ADMIN_EMAIL_ADDRESS`                                                                     |                                                | Comma separated list of email addresses of the admin users ID                                                                             |
 | **Azure Cognitive Search is optional. This is only required for chat over file feature.** |
 | `AZURE_SEARCH_API_KEY`                                                                    |                                                | API Key of Azure Cognitive search                                                                                                         |