Just better wording.
Since some of the ioctl calls do not take arguments, I suggests "for ioctl calls that takes an argument, the following structure is used:"
The .Fa buf field points to a buffer containing a packed nvlist. The .Fa nbytes field indicates the size of the packed nvlist if a packed nvlist is to be submitted to the subsystem, or the size of the buffer if a packed nvlist is to be retrieved from the subsystem.
And for explaining what nvlist is we should leave that to nv(9).