Docs: improve installation/ubuntulinux memory and swap accounting section

Docker-DCO-1.1-Signed-off-by: Jean-Baptiste Barth <jeanbaptiste.barth@gmail.com> (github: jbbarth)
Merge pull request #5178 from jbbarth/docs-improve-ubuntulinux-accounting-section

Docs: improve installation/ubuntulinux memory and swap accounting section