[InterNetNews/inn] 3de00c: Document the exclusion of control groups in newsfeeds

[Julien ÉLIE]

  Branch: refs/heads/2.6
  Home:   https://github.com/InterNetNews/inn
  Commit: 3de00c19ba849e385dd0826860e1b47dc20baeb5
      https://github.com/InterNetNews/inn/commit/3de00c19ba849e385dd0826860e1b47dc20baeb5
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2022-02-05 (Sat, 05 Feb 2022)

  Changed paths:
    M doc/pod/newsfeeds.pod

  Log Message:
  -----------
  Document the exclusion of control groups in newsfeeds

Mention it in a second example at the beginning of the man page,
as it is a common use case.