doc: update linux update/upgrade instructions

Linux package update/upgrade instuctions were incomplete (on
Ubuntu the instructions updated the local database but didn't
actually do the upgrade).  On Fedora, the dnf update command was
deprecated in favor of dnf upgrade.

Change-Id: I83e126cf924d9c3720fcfb77b0d5abda943a6af5
Jira: ZEP-516
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>