cl-extra: Fix docstring retrieval

* lisp/emacs-lisp/cl-extra.el (cl--describe-class-slots): Fix
docstring retrieval (bug#46662).
5 jobs for master in 32 minutes and 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prep Images
passed #13465
prep-image-base

00:00:38

 
  Build Images
passed #13466
build-image-inotify

00:22:17

 
  Fast
failed #13467
test-fast-inotify

00:09:49

 
  Normal
skipped #13468
test-lisp-inotify
skipped #13469
test-lisp-net-inotify
 
Name Stage Failure
failed
test-fast-inotify Fast
1 files contained unexpected results:
misc/test-custom-libs.log
Makefile:340: recipe for target 'check-doit' failed
make[1]: Leaving directory '/checkout/test'
make[1]: *** [check-doit] Error 1
Makefile:310: recipe for target 'check' failed
make: Leaving directory '/checkout/test'
make: *** [check] Error 2
ERROR: Job failed: exit code 2