DOC-8666 Document that setting-saslauthd is a GNU/Linux only

Change-Id: I17ea6c78c42c427f5d57e7d0aa154daa0d4dc516
Reviewed-on: https://review.couchbase.org/c/couchbase-cli/+/167068
Reviewed-by: Maksimiljans Januska <maks.januska@couchbase.com>
Reviewed-by: Patrick Varley <patrick@couchbase.com>
Tested-by: James Lee <james.lee@couchbase.com>

Author: jamesl33