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