scala_specs: mention specs lib in the docstring; mention junit works, too

because not all Scala tests are scala_tests

Bugs closed: 622

Reviewed at https://rbcommons.com/s/twitter/r/1326/
Sync of https://github.com/pantsbuild/pants/commit/68c2f6b5c1661fd7661523cad9c22a554a2c59f4