[Edu4rdSHL]

• hugo.yaml and reference.md: documented version 2.4.0.
• reference.md: the -p synopsis and row (no host port or 0 takes a free one, a host range with one container port its first free one, the long form), and the --dns-search and --add-host rows.
• networking.md, Published ports: free host ports, host ranges and the long form; the machine keeps the ports it got.
• images.md: how pull picks the entry of an image index.

Branched from docs-2.3.2, so it carries #68's commit as well.