Fix behavior of gnus-summary-very-wide-reply with prefix arg

Fix behavior of gnus-summary-very-wide-reply with prefix arg

* lisp/gnus/gnus-msg.el (gnus-summary-very-wide-reply): If a prefix
argument has been given, the value of YANK will be a list containing the
current article number.  This should not be used to retrieve a number of
work articles; that should be derived from the value of the
current-prefix-arg (or marked articles).
* doc/misc/gnus.texi: The interplay of prefix arg and marked articles is
complex; attempt to clarify.

(cherry picked from commit 7a0ee5d65f214102734dd22edb641b164a1b73af)