[Handbook]: Clarify kernel modules on upgrades

Forewarn users that FreeBSD-provided packages of kernel modules might be
unusable with a new version of the OS until some time after the version is
announced.

Approved by: doceng@ (carlavilla@, fernape@)
Reviewed By: ceri@, jrm@
Differential Revision: https://reviews.freebsd.org/D39474