[CFD] changes to Handbook chapter on Ports

[Mark Linimon]

Executive summary: the chapter needed help, and still needs more.

Full explanation:

I have submitted https://reviews.freebsd.org/D9124 to add a section
about ports-mgmt/synth, and a compare-and-contrast table for it and
poudriere, portupgrade, and portmaster.  I fully expect this section
to be controversial.

My aim is twofold:

 - try to make our documentation as useful for our users as possible
   (while still trying to be succinct)

 - try to meet the objections that have arisen about its favorable
   treatment of portupgrade and portmaster.

I don't think the Handbook is the place for a full discussion of
the controversy around these tools (see "succinct").  I'll leave the
judgement about whether that should be a wiki entry, or a new Article,
to someone else.

When I started working on this task, I found that the whole chapter
was stale and sometimes awkward.  I submitted changes to Phabricator as:

  https://reviews.freebsd.org/D8838
  https://reviews.freebsd.org/D8839

which now correspond, respectively, to:

  https://svnweb.freebsd.org/doc?view=revision&revision=49827
  https://svnweb.freebsd.org/doc?view=revision&revision=49828

Although the reviews were made public, only my doc mentor (wblock)
commented.

A followup commit:

  https://svnweb.freebsd.org/doc?view=revision&revision=49829

was only reviewed between he and I.

I am somewhat hopeful that the above commits will invite minimum
controversy.  However, I am reasonably confident that D9124 will.

And now I'll sit back and await feedback.

References: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214679

mcl