bsdinstall.8: Clarify the description of ZFSBOOT_FORCE_4K_SECTORS

This variable does not set the exact sector size of the pool, but
controls the minimum sector size. The sector size of the underlying
disks can always be larger than the minium controlled by this knob.

PR: 274513
Reported by: Albin "a12l" Otterhäll <bugs.freebsd.org@a12l.xyz>