List increased memory use in upgrading document

The 2.3 additions could be perceived as a memory leak so a note in the
upgrading document could be helpful.

Documentation/Upgrading/Upgrading-To-MaxScale-2.3.md

@@ -9,12 +9,30 @@ to the [ChangeLog](../Changelog.md).
 Before starting the upgrade, we recommend you back up your current
 configuration file.
 
-### Unknown global parameters
+## Increased Memory Use
+
+Starting with MaxScale 2.3.0 up to 40% of the memory can be used for
+caching parsed queries. The most noticeable change is that it improves
+performance in almost all cases where queries need to be parsed. Most of
+the time this happens when the readwritesplit router or filters are used.
+
+The amount of memory that MaxScale uses can be controlled with the
+`query_classifier_cache_size` parameter. For example, to limit the total
+memory to 1GB, add `query_classifier_cache_size=1G` to your
+configuration. To disable it, set the value to `0`.
+
+In addition to the aforementioned query classifier caching, the
+readwritesplit session command history is enabled by default in 2.3 but is
+limited to a maximum of 50 commands after which the history is
+disabled. This is unlikely to show in any metrics but it contributes to
+the increased memory foorprint of MaxScale.
+
+## Unknown Global Parameters
 
 All unknown parameters are now treated as errors. Check your configuration for
 errors if MaxScale fails to start after upgrading to 2.3.1.
 
-### `passwd` is deprecated
+## `passwd` is deprecated
 
 In the configuration file, passwords for monitors and services should be
 specified using `password`; the support for the deprecated