Document slow option in etc/object-server.conf

Change-Id: Ic9940b0b830a468887878f7b0d7ca42c2cbbebd5

etc/object-server.conf-sample

@@ -82,6 +82,10 @@ use = egg:swift#object
 # set log_address = /dev/log
 #
 # max_upload_time = 86400
+#
+# slow is the total amount of seconds an object PUT/DELETE request takes at
+# least. If it is faster, the object server will sleep this amount of time minus
+# the already passed transaction time.
 # slow = 0
 #
 # Objects smaller than this are not evicted from the buffercache once read