Clarify output order

[ejmahler]

Fixes #104

[HEnquist]

Item 3 has the direction flipped, the negative frequencies come out in descending magnitude. Bins 4 and 5 of a 6-point transform are -2 and -1, so the one closest to zero is last.

On length, subjective of course, but I think a concrete example earns the two lines. It makes the even-length middle bin obvious in a way more words don't:

//! Elements in the output have the following order:
//!
//!  1. Frequency zero
//!  2. Positive frequencies, from smallest magnitude to largest
//!  3. Negative frequencies, from largest magnitude to smallest
//!
//! For example, with 6 elements and a sample rate `fs`, the output frequencies are
//! `[0, fs/6, 2*fs/6, 3*fs/6, -2*fs/6, -fs/6]`, where `3*fs/6` is the Nyquist frequency.

I also numbered the list 1/2/3 instead of three times 1.. It renders the same, but these are doc comments that get read in the source more often than on docs.rs.