Update to r49829:

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.