jar_library docstring += dependencies

eagle-eyed J.I. Ioannidis noticed that our 3rdparty JVM docs show jar_library(dependencies=) but the BUILD dictionary doesn't (at least, I think that's what's going on)

until we fix https://github.com/pantsbuild/pants/issues/142, add doc for this param by hand, like an animal

Reviewed at https://rbcommons.com/s/twitter/r/960/