[InterNetNews/inn] a37c29: Fix expire documentation

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: a37c29fe55cd28a648498e61c27c80b937ba15d9
      https://github.com/InterNetNews/inn/commit/a37c29fe55cd28a648498e61c27c80b937ba15d9
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2022-07-10 (Sun, 10 Jul 2022)

  Changed paths:
    M doc/pod/expire.pod

  Log Message:
  -----------
  Fix expire documentation

What really does expire was previously wrongly documented.  Removal of
overview data was mentioned, and how groupbaseexpiry works was not clear.

Hopefully this rewrite will permit a better understanding of the process of
article expiration.