; Warn of while/dolist pitfall in gnus-sum.el

Suggested by Andy Moreton in the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-04/msg00294.html
* lisp/gnus/gnus-sum.el (gnus-summary-move-article): Add comment
warning of common while/dolist pitfall. (bug#33653#134)
1 job for master in 60 minutes and 2 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #1285
test-all

01:00:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds