freebsd-update(8): Explain conflicts in /etc

Unlike etcupdate(8), freebsd-update only prints the files with
conflicts. It doesn't merge.

PR: 204549
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/826

(cherry picked from commit a8c1ffca7a594733a91c1513d0185be174d1115f)