WIP2, automagic docstrings for notmuch-emacs.rst

[David Bremner]

This is a big improvement over
id:20180527185530.6795-1-david at tethera.net since it doesn't require
any source code modifications to generate the docstrings. It also
fixes the whining from the byte-compiler, and works for function
docstrings as well as variable docstrings.

The pseudo markup using |br| and |indent| is kindof gross still, but I
guess I can live with it.

I did test building html docs (make sphinx-html) and thost seem to
work OK as well, although not installed anywhere by default.

The use of docstrings in the manuals is still not guarded properly for
the no-emacs case; presumably we'd want the other documents to still
build.

I'm also not really sure about what sphinx markup to use for
keybindings with functions (see the "Copy to kill ring" section)