bug#52969: Shortdoc "string" group leads to backtrace

[Stefan Kangas]

Stefan Kangas <stefan@marxist.se> writes:

> On master, emacs -Q:
>
>     M-x shortdoc-display-group RET string RET
>
> Debugger entered--Lisp error: (wrong-type-argument stringp nil)
>   split-string(nil "\n")
>   shortdoc--display-function((truncate-string-to-width :eval

We get nil here because on master, after emacs -Q:

    (documentation 'rx)
    => nil

Whereas on the emacs-28 branch:

    (documentation 'rx)
    => "Translate regular expressions REGEXPS ..."

Bisecting points to this commit:

    59732a83c8875c8986d2221600d559a24d8309cc is the first bad commit
    commit 59732a83c8875c8986d2221600d559a24d8309cc
    Author: Stefan Monnier <monnier@iro.umontreal.ca>
    Date:   Thu Dec 30 23:17:45 2021 -0500

        Don't store docstrings of preloaded .el files in etc/DOC

This always works, even with that commit:

    (progn (rx "foo")
           (documentation 'rx))
    => "Translate regular expressions REGEXPS ..."