Here's a new version of the patch, with the fixes that Ned pointed out.

I also tried to address concerns about lost information; menu divisions have been added to Doc/library/idle.rst, along with the blurb about running without a subprocess being deprecated.  Also, every instance of :kbd:`C-x` has been expanded to :kbd:`Control-x`, as that's how help.txt has commands written.  A rather unrelated change that I snuck in while I was editing idle.rst was to move the index markers for Class browser and Path browser to be above those entries rather than below.

The generated help.txt is significantly longer than the old version but I don't think that's all bad.  Most of the extra lines are new whitespace or things that had been a single line being broken up into two.  I personally thought the old help.txt was rather too dense in some places, though I might agree that the generated version is a bit sparse in others.

The paragraph about environment variables does have a rather unfortunate number of backticks, but I don't think it's unreadable.  It's also information that wasn't present in the original help.txt.

Thank you for the review, Ned :)