Update Apple descriptions in the handbook.

It has been a while since Apple changed their mac operating system's
name from 'OS X' to 'macOS'. Also, iOS, iPadOS, watchOS, tvOS, and
bridgeOS also use Apple Darwin, which borrowed some components from
FreeBSD.

Reviewed By: gjb, pauamma

Approved by: gjb

Differential Revision: https://reviews.freebsd.org/D34385