[3.12] gh-56374: Clarify documentation of nonlocal (GH-116942) (#117023)

Define 'nonlocal scopes' in a way that excludes class scopes.
Rearrange the rest of the doc.  Add "Programmer's note".

(cherry picked from commit 025ef7a5f7b424fba8713e448244b952bf897df3)
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>