Clean up docs for macOS defaults. Simply use "Unix" across the board
instead of being inconsistent and occasionally using "Unix, macOS". Also
remove stale defaults that were erroneously renamed to "macOS" from
"Macintosh" when they were actually referring to Mac OS 9 which is no
longer supported.
closes: #15924
Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>