Checkdoc fixes in `gnus/*.el`

Checkdoc fixes in `gnus/*.el`

* lisp/gnus/gnus-agent.el (gnus-agent-enable-expiration)
(gnus-agent-with-refreshed-group, gnus-agentize)
(gnus-agent-fetch-headers, gnus-agent-update-files-total-fetched-for)
(gnus-agent-update-view-total-fetched-for):
* lisp/gnus/gnus-art.el (gnus-article-address-banner-alist)
(gnus-default-article-saver, gnus-mime-view-part-as-charset)
(gnus-block-private-groups, gnus-article-edit-mode):
* lisp/gnus/gnus-dired.el (gnus-dired-mode, gnus-dired-attach)
(gnus-dired-find-file-mailcap, gnus-dired-print):
* lisp/gnus/gnus-msg.el (gnus-summary-attach-article):
* lisp/gnus/gnus-score.el (gnus-home-score-file):
* lisp/gnus/gnus-search.el (gnus-search-ignored-newsgroups)
(gnus-search-mu-switches, gnus-search-mu-remove-prefix)
(gnus-search-thread):
* lisp/gnus/gnus-sieve.el (gnus-sieve-string-list):
* lisp/gnus/message.el (message-beginning-of-line):
* lisp/gnus/mm-url.el:
* lisp/gnus/mm-view.el (mm-w3m-setup, mm-setup-w3m):
* lisp/gnus/mml-sec.el (mml-signencrypt-style):
* lisp/gnus/nndiary.el:
* lisp/gnus/nnimap.el (nnimap-server-port, nnimap-use-namespaces)
(nnimap-expunge):
* lisp/gnus/nnmail.el (nnmail-incoming-coding-system):
(nnmail-ignore-broken-references):
* lisp/gnus/nnmairix.el (nnmairix-default-group):
* lisp/gnus/nntp.el (nntp-report):
* lisp/gnus/nnvirtual.el (nnvirtual-update-read-and-marked):
(nnvirtual-partition-sequence):
* lisp/gnus/spam-stat.el (spam-stat-process-directory-age)
(spam-stat-last-saved-at, spam-stat-save):
* lisp/gnus/spam.el (spam-enter-ham-BBDB, spam-check-BBDB):
Checkdoc fixes.

(cherry picked from commit e19fd6a22de00465b8fa6db574d71d03fa37b11f)