When quoting single-quotes in docstring like shown below, the
closing quote-form "\\=" gets (font-lock-constant-face
font-lock-doc-face), but should have no face.
(defvar foo nil
"\\='a\\='")
GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
2.24.32, cairo version 1.16.0) of 2022-05-26