don't hide docs for exclusives= target param in an unpublished docstring.

A blurb about how to use exclusives was hiding in the CheckExclusives goal doc. Many BUILD dictionary entries mentioned this, but weren't high-tech enough to "hyperlink" that doc.

So... move the "user-facing" exclusives doc out of tasks/check_exclusives.py into docs/build_files.rst.

in many backend/.../targets/*.py files, link to the new location.

Testing Done:
generated doc set, eyeballed

to make sure my automated comment-fixup doesn't destroy the world's syntax, kicked off https://travis-ci.org/pantsbuild/pants/builds/32287735

Bugs closed: 451

Reviewed at https://rbcommons.com/s/twitter/r/865/