Add Task.workdir docs.

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/39909933

Bugs closed: 766

Reviewed at https://rbcommons.com/s/twitter/r/1277/
Sync of https://github.com/pantsbuild/pants/commit/699f191fac38d5a782997206066be4be415bd3e9