#442 add 'update docstring' quick-fix if docstring already exists

[Glyph Lefkowitz]

Currently, if you invoke the 'make docstring' quick-fix multiple times, you get multiple docstrings.

However, if I define a function with signature 'def x()', make a docstring, write about it, then decide to modify the signature to 'def x(a, b, c)', what I really want the quick-fix to do is update the docstring to add '@param' (and possibly '@type') epytext declarations inside the *existing* docstring, not to add another one.