Add a short section 4.5.3.1, "Tools To Upgrade And Manage Ports".
This removes the assertion that only portupgrade or portmaster
are suitable for upgrading ports, and provides a framework for
discussing more alternatives as they become available.

Synth is mentioned as an alternative. A subsequent patch will
be put up for community discussion.

While here, clean up some duplicate and awkward wording.

This commit expands on the rework done in D8838 and D8839.

Submitted by: linimon
Reviewed by: wblock
Approved by: wblock (mentor)