The ERRORS section for aio_{fsync,mlock,read,write} mention ENOSYS (aio(4) support in optional), but aio_{cancel,error,return,suspend,waitcomplete} don't.
I confirmed my assertion with the attached non-ATF testcase on FreeBSD 11-CURRENT without aio kldload'ed. I'll try to add an ATF testcase for it.
Created attachment 143692 [details] Patch to manpages noting missing ENOSYS requirement
Created attachment 143693 [details] non-ATF ENOSYS testcase
MARKED AS SPAM
Hello! Adding these errors and organization seems sane to me on 15-current, can you commit this? I don't have my computer to see if it still applies cleanly.