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

[Stefan Monnier]

> We get nil here because on master, after emacs -Q:
>
>     (documentation 'rx)
>     => nil

Oh, indeed we now drop the docstrings from the autoloads in
`lisp/loaddefs.el`.  I see a few ways out of that:

1- Forget about autoloads's docstrings altogether (i.e. always load the
   autoload's target when we need the docstring).
2- Revert part of 59732a83c8 so that we still keep the docstrings of
   `lisp/loaddefs.el` in etc/DOC.
3- Byte-compile `lisp/loaddefs.el`.

I pushed a patch which does n°2.


        Stefan