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.
As long as we don't use Docker's restart, their ID is sufficient.  If we decide we want to use their restart logic, we'll probably have to pressure them for some other bit of info that we can capture to fill this need.  I don't see the point of adding spec and code at this point to take over something that works and is already sufficiently abstract.

In other words, we can always add code later..