bug#51852: Gnus: Add User-Defined Spec example

[積丹尼 Dan Jacobson]

(info "(gnus) Formatting Basics") says

   Also Gnus supports some extended format specifications, such as
‘%&user-date;’.

OK, please say what user-date is, and list the rest.

Wait, on (info "(gnus) User-Defined Specs") we see it is something we
create ourselves. So on (info "(gnus) Formatting Basics") the "foo"
example should be used, and not "user-date".

And mainly, on (info "(gnus) User-Defined Specs") there is no example of
how to make a User-Defined Spec. E.g., how to continue the example on
(info "(gnus) Advanced Formatting") to finally get 96/08/09 .

By the way, that (info "(gnus) Advanced Formatting") example,

       Let’s take an example.  The ‘%o’ spec in the summary mode lines will
    return a date in compact ISO8601 format—‘19960809T230410’.  This is
    quite a mouthful, so we want to shave off the century number and the
    time, leaving us with a six-character date.  That would be ‘%~(cut-left
    2)~(max-right 6)~(pad 6)o’.  (Cutting is done before maxing, and we need
    the padding to ensure that the date is never less than 6 characters to
    make it look nice in columns.)

is bad programming: not Y10K proof.