Improve docstrings

No change in the actual code.  Changes are only in docstrings.  I hope this increases hackability of Jedi code base.

It would be really nice if all non-trivial functions in Jedi has doctest snippets so that you can start playing with it at once.  It also really helps you to understand what function does.  So please add more!  For example, I'd like to see a good example for `evaluate.find_name`.