update emacs section in editor.md

This merge request consists of two improvements over current emacs setup:

1. The math symbol lists are added to rules in reverse order to prioritize the upright fonts. See discussion on mattermost here for more details.
2. Removing the comments about customize-set-value, since Emacs source code suggest that set-input-method already sets default IME.

I didn't test the elisp scripts on a fresh emacs setup, but they work in my emacs and doesn't seem to have dependencies outside math-symbol-lists.