Clarifying pagerduty

Should help clear up the following internal issues: https://dev.gitlab.org/gitlab/organization/issues/651
https://dev.gitlab.org/gitlab/organization/issues/666
https://dev.gitlab.org/gitlab/organization/issues/667

\cc @balameb @kelvinmutuma @cabargas @patricio @dblessing @pcarranza @marin @jacobvosmaer @tmaczukin @jnijhof @sytses