style.9: clarify FALLTHROUGH

FALLTHROUGH is intended for a block of code that cascades to the next
case block. Multiple case statements sharing a single block of code do
not need a FALLTHROUGH comment.

Reviewed by: imp, markj, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47242

(cherry picked from commit d55d5dd9f7adcffa6a1f2a49956d7fd7d549aba1)