doc: Mention archive_command failure handling on signals

The behavior is similar to restore_command, which was already documented
for the restore part, but not the archive part.

Author: Benoit Lobréau
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAPE8EZ7akCzc1hWohA4AcbmKtHh9rcWAB5MStOeZD2+9jC+hLQ@mail.gmail.com

doc/src/sgml/backup.sgml

@@ -639,6 +639,15 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/0
     it will try again periodically until it succeeds.
    </para>
 
+   <para>
+    When the archive command is terminated by a signal (other than
+    <systemitem>SIGTERM</systemitem> that is used as part of a server
+    shutdown) or an error by the shell with an exit status greater than
+    125 (such as command not found), the archiver process aborts and gets
+    restarted by the postmaster. In such cases, the failure is
+    not reported in <xref linkend="pg-stat-archiver-view"/>.
+   </para>
+
    <para>
     The archive command should generally be designed to refuse to overwrite
     any pre-existing archive file.  This is an important safety feature to