Document some user-settable make variables in ports.7

This is not a comprehensive list, as the variables themselves are spread
out over multiple files, but it is a start.

Add a section to make.conf noting that variables may be set there that
affect ports builds, but refer to ports.7 and elsewhere for the actual
listing; any listing in make.conf.5 would likely become out of date
fairly quickly.

PR: docs/189199
Reviewed by: bdrewery (previous version)
Approved by: hrs (mentor)