PEP 649 update #187

We have some post-acceptance updates for 649, rolled up into one PR:

• Lots of clarification about intent--not changes, just better expressing the goal.
• Replace some unnecessary implementation details (that may change anyway) with descriptions of the desired outcome/behavior.
• Change the VALUE / FORWARDREF / STRING format constants to members of an IntEnum, which also means we can get rid of FORMAT_MIN and FORMAT_MAX. A win all around.

My understanding is, such changes in an already-accepted PEP need SC signoff. The PR is ready for your inspection. Thanks!