Re: archive modules

I noticed that this patch has gone around and mostly purged mentions of
archive_command from the documentation and replaced them with
archive_library.  I don't think this is helpful, since people still use
archive_command and will want to see what the documentation has to say
about it.  I suggest we rewind that a bit and for example replace things
like

     removed or recycled until they are archived. If WAL archiving cannot keep up
-   with the pace that WAL is generated, or if <varname>archive_command</varname>
+   with the pace that WAL is generated, or if <varname>archive_library</varname>
     fails repeatedly, old WAL files will accumulate in <filename>pg_wal</filename>

with

     removed or recycled until they are archived. If WAL archiving cannot keep up
     with the pace that WAL is generated, or if <varname>archive_command</varname>
     with the pace that WAL is generated, or if <varname>archive_command</varname>
     or <varname>archive_library</varname>
     fail repeatedly, old WAL files will accumulate in <filename>pg_wal</filename>