I disagree.  Purpose of __doc__ is to explain functionality all at once.
This command idiom is useful:

$ python -c 'from a_module import thing; help(thing)'

The doctest module is a lightweight nicety that helps verify that which
is suitable.  The sufficiently simple algorithms of my code have doc
strings that are the complete test and explanation.  For others I
provide both docstring and unit tests.  But with many I explain the
arguments and output, possibly the algorithm in a doc string.  Tests and
use case examples reside in the module's unit test.

I'm among the "Choose correct tool for the job.  python comes with full
tool bag." group.