Merge from origin/emacs-26

e7818cb Fix nnmail-expiry-wait docs and custom :types
8b7c776 * lisp/simple.el (kill-do-not-save-duplicates): Doc fix.  (Bu...

diff --cc doc/misc/gnus.texi
Simple merge

diff --cc lisp/gnus/gnus-cus.el
Simple merge

diff --cc lisp/gnus/nnmail.el
Simple merge

diff --cc lisp/simple.el
Simple merge