They are both used for determining a k-value at which the f(k) in fourier space falls below some threshold.  If `f(k) < kvalue_accuracy`, then we are allowed to substitute `f(k) = 0`.  This is usually implemented by finding some `k` above which all `f(k)` satisfy this and then then condition on `if k > k_max...`.  

`maxk_threshold` is operationally similar, since it is used to find some `k` above which all `f(k) < maxk_threshold`.  This is where we allow the fft to be truncated.  It would be kind of silly to have the accuracy parameter larger than the threshold parameter, since it would mean we would be building a large fft, but approximating a bunch of those values to be 0.  That's what I meant by "more stringent".

However, if this is either too obvious or too confusing or just plain unhelpful, I don't mind removing this comment.