man tuning comes with the OS, it contains information that may be out of date, and generally not a good idea to provide advice which can be reversed or negated with incremental updates.  There are plenty of tuning guides not included with the OS, but when one is included with the OS it must be up to date.

Example the following excerpt seems to come from the 4.x days, certainly it is not include or allude to the suggestions to improve performance under ZFS:

<exceprt man tuning FreeBSD 11.0-RELEASE-p8           May 9, 2016>
STRIPING DISKS
     In larger systems you can stripe partitions from several drives together
     to create a much larger overall partition.  Striping can also improve the
     performance of a file system by splitting I/O operations across two or
     more disks.  The gstripe(8), gvinum(8), and ccdconfig(8) utilities may be
     used to create simple striped file systems.  Generally speaking, striping
     smaller partitions such as the root and /var/tmp, or essentially read-
     only partitions such as /usr is a complete waste of time.  You should
     only stripe partitions that require serious I/O performance, typically
     /var, /home, or custom partitions used to hold databases and web pages.
     Choosing the proper stripe size is also important.  File systems tend to
     store meta-data on power-of-2 boundaries and you usually want to reduce
     seeking rather than increase seeking.  This means you want to use a large
     off-center stripe size such as 1152 sectors so sequential I/O does not
     seek both disks and so meta-data is distributed across both disks rather
     than concentrated on a single disk.  If you really need to get
     sophisticated, we recommend using a real hardware RAID controller from
     the list of FreeBSD supported controllers.