manpages on the website for other operation systems is confusing.

I've experienced similar before, but this time I was talking to someone who
told me that ionice does indeed exist on FreeBSD - he googled it.

Here are the top 3 result of googling "freebsd man ionice":

 | ionice(1)
 | 
 | FreeBSD Manual Pages
 | https://man.freebsd.org › cgi › man › manpath=Ubuntu+...
 | IONICE(1) User Commands IONICE(1) NAME ionice - set or get process I/O scheduling class and priority SYNOPSIS ionice [-c class] [-n level] [-t] -p PID ionice ...
 | 
 | ionice(1)
 | 
 | FreeBSD Manual Pages
 | https://man.freebsd.org › cgi › man › manpath=SuSE+11
 | ionice(1) General Commands Manual ionice(1) NAME ionice - get/set program io scheduling class and priority SYNOPSIS ionice [[-c class] [-n classdata] [-t]] ...
 | 
 | ionice(1)
 | 
 | FreeBSD Manual Pages
 | https://man.freebsd.org › cgi › man › manpath=CentOS+7
 | IONICE(1) User Commands IONICE(1) NAME ionice - set or get process I/O scheduling class and priority SYNOPSIS ionice [-c class] [-n level] [-t] -p PID... ionice ...

Now, looking carefully, you can see the mention of the various linux distributions,
but remember,the title "FreeBSD Manual Pages" is far bigger in graphical browsers.

I don't know why you'd want linux (and other OS) manpages on the FreeBSD
site, but I'd suggest that you at least change the page title to something
more descriptive, and also set the norobots meta tag and/or robots.txt to
ensure these pages aren't indexed by search engines.

Cheers, Jamie