Align test-gerrit and stages.yml by placing comments in stages.yml

Some, currently 4, jobs of stages.yml need special handling when used
in test-gerrit. A special case handling is implemented in test-gerrit.

With this change it is made clear to have a look at test-gerrit for the
details about these 4 jobs

CMK-38043

Change-Id: I536c861cf6ff70c6cf529ea8f23d8865dd3988b6