build.7: Improve make delete-old example

Be more explicit that deleting old files and libraries needs to be
really after you're rebooted a third time and you're really actually
sure your applications aren't using old libs, following irc discussion.

Also adjust an example slightly to fit on 80 character console.

MFC after: 3 days
Discussed with: emaste, fuz, jhb
Differential Revision: https://reviews.freebsd.org/D54079