doc for transforms (q_to_tth, tth_to_q, etc.)

[yucongalicechen]

Problem

We kind of started this work in #178, but the functions have changed quite a lot since then. I think it's a good time to wrap this up.

Proposed solution

[yucongalicechen]

^not sure if we should put this in 3.6.0 release or 3.7.0 release

[bobleesj]

looks 3.6.0 to be .. :)