Add instructions to re-enable password auth

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -726,6 +726,18 @@ group.require_two_factor_authentication=false
 group.save
 ```
 
+## Authentication
+
+### Re-enable standard web sign-in form
+
+Re-enable the standard username and password-based sign-in form if it was disabled as a [Sign-in restriction](../../user/admin_area/settings/sign_in_restrictions.md#password-authentication-enabled).
+
+You can use this method when a configured external authentication provider (through SSO or an LDAP configuration) is facing an outage and direct sign-in access to GitLab is required.
+
+```ruby
+Gitlab::CurrentSettings.update!(password_authentication_enabled_for_web: true)
+```
+
 ## SCIM
 
 ### Fixing bad SCIM identities

doc/user/admin_area/settings/sign_in_restrictions.md

@@ -24,6 +24,8 @@ You can restrict the password authentication for web interface and Git over HTTP
 - **Web interface**: When this feature is disabled, the **Standard** sign-in tab is removed and an [external authentication provider](../../../administration/auth/index.md) must be used.
 - **Git over HTTP(S)**: When this feature is disabled, a [Personal Access Token](../../profile/personal_access_tokens.md) must be used to authenticate.
 
+In the event of an external authentication provider outage, use the [GitLab Rails console](../../../administration/operations/rails_console.md) to [re-enable the standard web sign-in form](../../../administration/troubleshooting/gitlab_rails_cheat_sheet.md#re-enable-standard-web-sign-in-form). This configuration can also be changed over the [Application settings REST API](../../../api/settings.md#change-application-settings) while authenticating with an administrator account's personal access token.
+
 ## Admin Mode
 
 > [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2158) in GitLab 13.10.