documentation

Pull request #2004: feature/DEV-3949-7.2-D10. fixed existing, added missing upgrade instructions (sub-issue 10).

Merge in WEB/documentation from feature/DEV-3949-7.2-D10 to release/7.2

* commit '5b064e7c558ef07312cf78dab76cf2055f2d519a':
  [DEV-3949] apt-get → apt (sub-issue 10)
  [DEV-3949] if upgrading agent/agent2, stop them too
  [DEV-3949] removed mention of RHEL8 (sub-issue 10)
  [DEV-3949] fixed existing, added missing upgrade instructions (sub-issue 10)