Merge branch 'docs-sso-enforce-note' into 'master'

Docs: SSO Groups: Add login session note

See merge request gitlab-org/gitlab-ce!29628

doc/user/group/saml_sso/index.md

@@ -35,6 +35,8 @@ SSO enforcement was:
 
 With this option enabled, users must use your group's GitLab single sign on URL to be added to the group or be added via SCIM. Users cannot be added manually, and may only access project/group resources via the UI by signing in through the SSO URL.
 
+However, users will not be prompted to log via SSO on each visit. GitLab will check whether a user has authenticated through the SSO link, and will only prompt the user to login via SSO if it has been longer than 7 days.
+
 We intend to add a similar SSO requirement for [Git and API activity](https://gitlab.com/gitlab-org/gitlab-ee/issues/9152) in the future.
 
 ### NameID