Fix docstrings referring to N_particles

[vyasr]

The docs currently have a random mix of terminology for the number of points. This issue is especially convoluted because ref_points need not coincide with points.

We should standardize the terminology around this and update all docstrings in freud to match. This is related to #179, and also somewhat to #180.

[vyasr]

I'm actually not 100% sure what to do with this. We've updated the majority of the APIs, and in particular removing the num_particles properties from most things has made this much easier. However, I'm not sure if we should update all the docstrings; some methods really are about particles. For example, does it make sense to change things in environment like BondOrder or LocalDescriptors to just discuss points in the docstrings? I'm not sure what the answer is, would like to discuss a bit more before deciding.

[bdice]

Docstrings are pretty good now. Closing as unnecessary.