DOC: Clean up the docstring for scipy.special.lambertw.
* Add the wikipedia article as a reference.
* Remove the **Properties** section--that would be more appropriate for a tutorial.
* Update the output of the examples to match the actual output.
* Added a `Returns` section.
* Removed the "TODO" note.
* A couple minor copy-edits.
Merge pull request #423 from WarrenWeckesser/lambertw-docstring

DOC: special: Clean up the docstring for scipy.special.lambertw.