bug#34473: Discuss "old/ancient" articles more

[積丹尼 Dan Jacobson]

In (info "(gnus) Read Articles") one reads about

‘O’
     Articles that were marked as read in previous sessions and are now
     “old” (‘gnus-ancient-mark’).


Well you need to say more:

* How do discover if one has "old/ancient" articles.
(Say tip: Use 99999 RET to enter a (e.g., nnml) group, instead of just RET.)

* What "old/ancient" articles are.

* How to prevent them from accruing.

Gnus v5.13
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian