Correct 'director/lib/enum_choice.py' docstrings partially
Correct 'director/lib/enum_choice.py' docstrings partially

wEdit docstrings in 'director/lib/enum_choice.py'.
Base the changes from Travis-CI checking:
[Travis-CI](https://travis-ci.org/stencila/hub/jobs/435452107#L818)

Address issue #123, partially.

stencila/hub,stencila/hub,stencila/hub,stencila/hub,stencila/hub