In particular, write down the rules for stack ownership because the
details of this are about to get very important with concurrent stack
shrinking. (Interestingly, the details don't actually change, but
anything that's currently skating on thin ice is likely to fall
through.)

Fox #12967.

Change-Id: I561e2610e864295e9faba07717a934aabefcaab9
Reviewed-on: https://go-review.googlesource.com/20034Reviewed-by: Rick Hudson <rlh@golang.org>