Update documentation editor setup instructions

Update documentation editor setup instructions

Now that the documentation sources are in XML rather than SGML, some of
the documentation about the editor, or more specifically Emacs, setup
needs updating.  The updated instructions recommend using nxml-mode,
which works mostly out of the box, with some small tweaks in
emacs.samples and .dir-locals.el.

Also remove some obsolete stuff in .dir-locals.el.  I did, however,
leave the sgml-mode settings in there so that someone using Emacs
without emacs.samples gets those settings when editing a *.sgml file.