Coding conventions for `defface`

[Tianxiang Xiong]

Many faces in Emacs Lisp end in `-face`, even though the Emacs Lisp manual section on `defface` says that they shouldn't: https://www.gnu.org/software/emacs/manual/html_node/elisp/Defining-Faces.html

One problem is that this convention is not mentioned in the Emacs Lisp Coding Conventions section of the manual: https://www.gnu.org/software/emacs/manual/html_node/elisp/Coding-Conventions.html#FOOT21

This can create confusion: https://github.com/clojure-emacs/cider/pull/1966#issue-216440476

I've added an item to the Coding Conventions section of the Emacs Lisp manual about naming faces.

Enclosed: patch

v1-0001-Add-item-to-Coding-Conventions-on-face-names.patch
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