Update internal/jepsen.md docs w/ recent test suite enhancements

We did a bunch of work on the jepsen tests back in december, let's update the docs to reflect the recent changes.

Still have some open questions for documenting or changing the behavior of the read consistency flags https://www.consul.io/docs/internals/consensus.html#consistency-modes. Will update here what the scope of work is when we've discussed internally more.