Updated docs/emacs

  * emacs.org: rewrote section about LaTeX exporting and previewing;
    added option 'org-use-sub-superscripts', set it to "'{}";
  * (TOP)/emacs.d/org-us.el: regenerated, tangled from Org document;
  * (TOP)/emacs.d/ac-comphist.dat: automatically updated.

Signed-off-by: Vladimir Lomov <lomov.vl@gmail.com>