Replace BULLET OPERATOR (∙) with BULLET (•)

Apparently, Emacs' built-in `tex` mode can do BULLET (`\bu` in
agda-mode), but not BULLET OPERATOR (`\.` in agda-mode).
Add an error message about BULLET OPERATOR

E.g., So that people coming from HoTT-Agda are less confused when using
agda-mode input.