Update to r40712:

Update the freebsd-update section:

Define minor and major version upgrades at the start of the section.
Mention that installed applications will still run after a minor
upgrade. Split the custom kernel portion into 8.X and earlier and 9.X
and later. Make the portupgrade instructions specific to major version
upgrades.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D3987