My placement of the note was carefully considered.  It is discussing the shell=False case and IMO belongs after that paragraph.  I understand now your concern about the note I omitted...and again I think this is a bug in the Popen API.  If shell=False, I think Popen should generate an error if args is a string.  Absent that fix, I've reworded the existing sentence along the lines you suggest, but using positive language rather than negative.  I've incorporated your fix for Eric's bug report, but I haven't added in the explicit references to the strings in the example.  IMO those are obvious and don't need repeating, but if others think it is clearer to add them, I'm fine with that.

As for subprocess, IMO there's no need to show the subprocess call.  The shlex result shows the list that would go into the subprocess call, and I think that's sufficient.  What do others think?

I've updated my version of the patch.