Add a note that the use of -B option does not guarantee a size of fragment
if -z option also used.

Recommend the use of zip(1) if compressed files of predictable size needed.

PR: docs/41089
Submitted by: Sevan Janiyan
Reported by: areilly@bigpond.net.au

While here, pet igor

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D18686