Add some more docstrings.

This PR adds some more docstrings to distarray, to get better coverage as per issue #525.

It also makes some pep8 adjustments, where I felt like following its rules. A lot of the added docstrings are for exception classes, and as I was investigating these, several seemed to be unused. In those cases, instead of adding a docstring (since there was no usage to suggest the correct text), I added a comment that the exception class seemed to be unused. If they really are unused, then perhaps they should just be removed from the code base. But I did not do that here so far.