bug#44707: 27.1; Add gnus-article-date-user user-defined examples

[積丹尼 Dan Jacobson]

(info "(gnus) Article Date") says

‘W T s’
     Display the date using a user-defined format
     (‘gnus-article-date-user’).  The format is specified by the
     ‘gnus-article-time-format’ variable, and is a string that’s passed
     to ‘format-time-string’.  See the documentation of that variable
     for a list of possible format specs.

Without examples the user is a loss at figuring out how to set this.

Maybe one does
(setq gnus-article-date-user "%a, %d ...")
but probably not.

(info "(gnus) User-Defined Specs")
has no examples either. In fact maybe some words are missing... without
examples one cannot be sure.

gnus-article-date-headers is a variable defined in ‘gnus-art.el’.
its docstring seems to say we can do
(set-variable 'gnus-article-date-headers '(user-defined))

Same on (info "(gnus) Customizing Articles")

Maybe we are supposed to do
(set-variable 'gnus-article-date-headers '("%a, %d...")) but that
triggers an error.

Wait. Let's try
(set-variable 'gnus-article-time-format "%a, %d...")
(set-variable 'gnus-article-date-headers '(user-defined))

Ah, finally I figured out what the docs are trying to say. Wish they
added an example.

Alas, not only does doing that correctly affect W T s itself,
but it also affects all the other W T's!

emacs-version "27.1"