Improve basis docstrings

[billbrod]

As described in #50 , I think basis docstrings should have a line or two before parameters that describe what the basis looks like and when you would use them. E.g., do they tile uniformly, are they each about the same size, what types of variables make sense (visual input, head direction, etc.). And then the footnote to references belongs in those lines (it looks weird in the docstring in jupyter or whatever to have the link right at the top).

Our documentation will have a longer discussion of the differences between the different basis functions, when to use them, etc., but something quick to give people some guidance will be helpful.