This revision updates all of the internal xrefs in this chapter to use the syntax instructed in Asciidoctor official doc on xrefs because crossref: is (uncrossreferencable) broken on pdf and possibly elsewhere.
Related bug : [https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266107]
Similar fix for advanced networking chapter : [https://reviews.freebsd.org/D46258]
Asciidoctor doc reference [https://docs.asciidoctor.org/asciidoc/latest/macros/xref/]