Provides documentation for some of the properties and methods in target.py

The type annotations help when using an IDE like IntelliJ

Testing Done:
There should be no code changes.  running CI

Bugs closed: 707

Reviewed at https://rbcommons.com/s/twitter/r/1196/
Sync of https://github.com/pantsbuild/pants/commit/ef1ac14d6438c6ad538b6c99f2cfd5291321deca