15.0/relnote: Organize Upgrading section

Judging from the mailing list discussion, this section needs organized
better. While here, sneak in a bit of wordsmithing. We still have a lot
of work to do improving the instructions for upgrading systems installed
with packages, but we have to keep moving.

Discussed with: cperciva (it's still ok to organize better)
Discussed with: ivy (s/distribution/& sets/)
Discussed with: jrm (upgrading from source wordsmithing)
Reviewed by: dch (previous, fixed a typo since)
Differential Revision: https://reviews.freebsd.org/D54181