setfib.2: Note that the number of FIBs can be adjusted after boot

Reviewed by: zlei, imp
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48545

(cherry picked from commit 010ee8215f5c899e23250828402af5b7bb354328)