Fix nnmail-expiry-wait docs and custom :types

* doc/misc/gnus.texi (Group Parameters, Expiring Mail):
* lisp/gnus/gnus-cus.el (gnus-group-parameters): Clarify
descriptions of nnmail-expiry, nnmail-expiry-wait, and
nnmail-expiry-wait-function.
* lisp/gnus/nnmail.el (nnmail-expiry-wait)
(nnmail-expiry-wait-function): Clarify docstrings and fix custom
:types (bug#36850).
1 job for emacs-26 in 28 minutes and 14 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #2865
test

00:28:14

 
Name Stage Failure
failed
test Test
Makefile:285: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check] Error 2
Makefile:255: recipe for target 'check' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:951: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1