Fix sphinx doc warnings

This fixes a few docstring bugs that were causing a series of sphinx build warnings. They are primarily pretty straightforward (although I did a few doc cleanup bits in `convolve` while I was there).

@mdboom and @keflavich - you might want to glance at this, because you were the last ones to touch the modified files (aside from one that I did), so you should tell me if I accidentally corrected something incorrectly.

@iguananaut - we'll want this in 0.2, but it's not crucial for beta 2