bug#51230: 29.0.50; `defvar-keymap' - wrong face after `:doc'

[Brahimi Saifullah]

(defvar-keymap my-mode-map :doc "My mode map")

The string after `:doc' has the font lock face for strings,
but it should instead have the one for documentation.

Keeping in mind the fact I'm not familiar with the intricacies of (e)lisp-mode,
I came upon the function `lisp-string-after-doc-keyword-p', which,
with a trivial change to the regexp, can fix this issue.

Yet I realized `defcustom' also has a `:doc' keyword,
which is properly fontified.  So maybe that functionality
can be leveraged somehow, and the approach above is not the right thing?