Fix doctests test path

While testing for #2512 and #2513 I noticed that the ``docs-path`` option to ``python setup.py test`` is not respected.  This is because it uses a relative path even when the docs build happens in a temporary directory that doesn't include the sphinx source.  So this just locks in the absolute path at the time the test command is triggered, rather then when the tests are actually running (which is in a separate process).

It also clarifies the help information for the ``-t`` option, which I realized interacts in a confusing way with the ``docs-path`` descritpion