BUG #15916: Confusing upgrade message

The following bug has been logged on the website:

Bug reference:      15916
Logged by:          Marnanel Thurman
Email address:      thomas@thurman.org.uk
PostgreSQL version: 11.4
Operating system:   Debian
Description:

This happens with version 11.4-1.pgdg90+1 in
http://apt.postgresql.org/pub/repos/apt/ .

The upgrade process prints this message:

     Success. You can now start the database server using:

         pg_ctlcluster 11 main start

But the server is restarted automatically, immediately afterwards. So the
message is confusing, and should be suppressed.