Allow the pool_recycle to be configured

Complete an outstanding todo.

Change-Id: I0ff7165d3f979c883733e1f438bb4b24569d290f
diff --git a/doc/source/admin/drivers/sql.rst b/doc/source/admin/drivers/sql.rst
index 34bb470..a269f5d 100644
--- a/doc/source/admin/drivers/sql.rst
+++ b/doc/source/admin/drivers/sql.rst
@@ -36,6 +36,13 @@
 
         GRANT ALL ON my_database TO 'my_user'@'%';
 
+   .. attr:: pool_recycle
+      :default: 1
+
+      Tune the pool_recycle value. See `The SQLAlchemy manual on pooling
+      <http://docs.sqlalchemy.org/en/latest/core/pooling.html#setting-pool-recycle>`_
+      for more information.
+
 Reporter Configuration
 ----------------------