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