This pull request should enable the use of autosummary directives for attributes that don't have a __doc__ attribute but a special comment (or a trailing docstring) as introduced by autodoc.
(At least) two things are maybe debatable:
I'm not quite sure why the autosummary extension has the imports from sphinx.ext.autodoc in the get_documenter function instead of at the top of the file. I left it this way but my change needs another symbol (the INSTANCEATTR singleton) from autodoc, I added this import to the top of the file.
I did not add any tests because I frankly do not now how this can be tested in a simple manner -- the autosummary extension did not contain any tests I could build upon...
This pull request should close issue #344, the example in that issue works as expected.