Update docstring for DataShape
Update docstring for DataShape

cc @chrish42

Regarding https://github.com/ContinuumIO/blaze/issues/807

ContinuumIO/datashape,quantopian/datashape,ContinuumIO/datashape,cpcloud/datashape,blaze/datashape,llllllllll/datashape,llllllllll/datashape,blaze/datashape,cowlicks/datashape,quantopian/datashape,cowlicks/datashape,cpcloud/datashape