Revisit identifiers spec

Partially addresses #1135.

This is the result of discussions in the past few days about imprecision and confusion. Hopefully simpler. There are a few FIXME comments in the doc - discuss.
LGTM, except I'd add a FIXME/TODO for whether AttemptID will always equal Docker ID.

FWIW, it looks like there is no longer a # 5, but I don't think that matters for the formatted text.