[docs] subprocess module constant documentation

[Skip Montanaro]

The subprocess module documentation mentions a lot of constants in
section 17.5.3.1. That is the only section labeled "Constants", though
it seems likely from the section hierarchy and the obscure names in
that section, that these are all Windows-specific constants. On my
Mac, I see DEVNULL, PIPE and STDOUT. While these are mentioned in
section 17.5.1, they aren't documented specifically as constants,
rather as "special value"s. I think the docs should have a single
"Constants" section one level up in the hierarchy from where it
currently live. It should describe all constants the user might
encounter, marking those which are platform-specific.

Thx,

Skip