Turn docstrings added in 6485315 into doctests.

I turned usage docstrings that were recently added into docstrings.  Which means that they're now run as part of the test suite.  This is useful, because it makes sure that usage examples are up-to-update and don't accidentally break.

It also allowed me to unearth two small bugs.  In the first, "set_param_values" was called in the usage docs instead of "set_all_param_values".  The other is the same as was fixed in #54.