0. emacs -Q
1. Type `C-h o fringe-mode RET' to pop up a *Help* buffer displaying
   help for both the function fringe-mode and the variable fringe-mode.
2. In the *Help* buffer type `c'.
=> Emacs dings and displays the message "No variable or face to customize"

When executing this recipe in emacs-30, after step 2 Emacs switches to a
*Customize* buffer for the variable fringe-mode, which is the expected
behavior after typing `c' (help-customize) in *Help*.  This is broken in
master.  The breakage is due to this commit:

commit e776df2a3eae0454ea85287e15ebba649bf8e918
Author:     Dmitry Gutov <dmitry@gutov.dev>
Commit:     Dmitry Gutov <dmitry@gutov.dev>
CommitDate: Sun Oct 6 04:32:00 2024 +0300

    help-setup-xref: Keep the local values only of some variables

    * lisp/help-mode.el (help-setup-xref): Kill all local variables,
    saving ones that are known to need to be preserved (bug#73637).

and the breakage remained after the two followup commits, which is the
current code state.

According to my debugging, calling help-setup-xref at the end of
describe-symbol after the function definition help has been added to the
*Help* buffer nullifies the property list of help-mode--current-data
(this happens on evaluating `(funcall major-mode)' in help-setup-xref
when the value of major-mode is 'help-mode), which renders
help-customize a noop.  The following patch fixes the breakage according
to my brief testing, though I'm not sure it doesn't have unwanted
side-effects (but at least I get the same test results on running make
check with the patch as without it).