[#1673] Change testing standards saying that not all methods need docstrings

If we follow the other coding standards, like making small tests, naming, etc.,
they should all be quite understandable just from the test name. So we leave
the option to add docstrings to the developer that's creating it, if she can't
make it simple enough not to need it.
Merge remote-tracking branch 'origin/1673-not-all-test-methods-need-docstrings'