Document protected manual actions feature

doc/ci/yaml/README.md

@@ -553,6 +553,8 @@ The above script will:
 #### Manual actions
 
 > Introduced in GitLab 8.10.
+> Blocking manual actions were introduced in GitLab 9.0
+> Protected actions were introduced in GitLab 9.2
 
 Manual actions are a special type of job that are not executed automatically;
 they need to be explicitly started by a user. Manual actions can be started
@@ -578,7 +580,9 @@ Optional manual actions have `allow_failure: true` set by default.
 
 **Statuses of optional actions do not contribute to overall pipeline status.**
 
-> Blocking manual actions were introduced in GitLab 9.0
+**Manual actions do inherit permissions of protected branches. In other words,
+in order to trigger a manual action assigned to a branch that the pipeline is
+running for, user needs to have ability to push to this branch.**
 
 ### environment