BUG/MINOR: doc: Be accurate on the behavior on pool-purge-delay.

Setting pool-purge-delay to 0 disables the pool, it doesn't keep the
connection forever, so correctly states this.

This should be backported to 1.9.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index a8022fc..88148c3 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -12000,8 +12000,8 @@
 
 pool-purge-delay <delay>
   Sets the delay to start purging idle connections. Each <delay> interval, half
-  of the idle connections are closed. 0 means it's never purged. The default is
-  1s.
+  of the idle connections are closed. 0 means we don't keep any idle connection.
+  The default is 1s.
 
 port <port>
   Using the "port" parameter, it becomes possible to use a different port to