postgresql.conf archive_command example

I think it would be useful to add the following explanation and sample
to the postgresql.conf sample file:

diff --git i/src/backend/utils/misc/postgresql.conf.sample w/src/backend/utils/misc/postgresql.conf.sample
--- i/src/backend/utils/misc/postgresql.conf.sample
+++ w/src/backend/utils/misc/postgresql.conf.sample
@@ -186,6 +186,9 @@#archive_mode = off            # allows archiving to be done                               # (change
requiresrestart)#archive_command = ''          # command to use to archive a logfile segment
 
+                               # placeholders: %p = path of file to archive
+                               #               %f = file name only
+                               # e.g. 'test ! -f /mnt/server/archivedir/%f && cp %p
/mnt/server/archivedir/%f'#archive_timeout= 0           # force a logfile segment switch after this
         # number of seconds; 0 disables
 
This corresponds to what we have in the documentation and mirrors the
example in recovery.conf.sample.

Objections?