Restore archive_command documentation

Restore archive_command documentation

Commit 5ef1eefd76f404ddc59b885d50340e602b70f05f, which added
archive_library, purged most mentions of archive_command from the
documentation.  This is inappropriate, since archive_command is still
a feature in use and users will want to see information about it.

This restores all the removed mentions and rephrases things so that
archive_command and archive_library are presented as alternatives of
each other.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/9366d634-a917-85a9-4991-b2a4859edaf9@enterprisedb.com