Fix minor defcustom issues in Gnus (Bug#66715)

Fix minor defcustom issues in Gnus (Bug#66715)

* lisp/gnus/gnus-art.el (gnus-button-prefer-mid-or-mail): Allow
function and add :tag to const values.
* lisp/gnus/gnus-bookmark.el (gnus-bookmark-bookmark-inline-details):
Fix docstring.
* lisp/gnus/gnus-sum.el (gnus-simplify-subject-fuzzy-regexp): Allow a
single regexp as value.
* lisp/gnus/message.el (message-indent-citation-function): Allow a
single function as value.
(message-mail-alias-type): Allow for a list of options as value.
(message-dont-reply-to-names): Allow a function as value.
* lisp/gnus/spam-report.el (spam-report-url-ping-function): Fix
default value for the function widget.