improve kerberos documentation

doc/integration/kerberos.md

@@ -9,7 +9,8 @@ Kerberos integration can be enabled as a regular omniauth provider, edit [gitlab
 
 You still need to configure your system for Kerberos usage, such as specifying realms. GitLab will make use of the system's Kerberos settings.
 
-The first time a user signs in with Kerberos credentials, GitLab will create a new GitLab user associated with the email, which is built from the kerberos username and realm. Existing GitLab users can go to profile > account and attach a Kerberos account.
+Existing GitLab users can go to profile > account and attach a Kerberos account. if you want to allow users without a GitLab account to login you should enable the option `omniauth_allow_single_sign_on` in config file (default: false). Then, the first time a user signs in with Kerberos credentials, GitLab will create a new GitLab user associated with the email, which is built from the kerberos username and realm.
+User accounts will be created automatically when authentication was successful.
 
 ### HTTP git access