doc: add/improve "Implemented by..." in generated docs

I didn't have a great answer for: "If this is intended only for BUILD file authors, then stick with jar; otherwise you might want to mention both [jar and pants.backend...JarDependency] for clarity." Here's an attempt to rectify that: With this change, the generated doc for "jar" mentions "JarDependency". And other thingies similarly mention where they're implemented.

This change adds lines to most of BUILD Dictionary items a la
  Implemented by pants.backend.python.targets.python_tests.PythonTests
...so if someone gives you a python_tests and you're a new Pants developer, you can find the underlying code without having to grep all over.

Also, where the Goals reference used to say
  Implemented by MinimalCover
...with this change it says
  Implemented by pants.backend.core.tasks.minimal_cover.MinimalCover

Testing Done:
./build-support/bin/publish_docs.sh -o

./pants tests/python/pants_test:all

Bugs closed: 187

Reviewed at https://rbcommons.com/s/twitter/r/479/