Add missing cache setting

Fix #1650 in 5.0.x

src/NHibernate/nhibernate-configuration.xsd

@@ -90,6 +90,15 @@
 								<xs:enumeration value="cache.region_prefix" />
 								<xs:enumeration value="cache.use_minimal_puts" />
 								<xs:enumeration value="cache.default_expiration" />
+								<xs:enumeration value="cache.use_sliding_expiration">
+									<xs:annotation>
+										<xs:documentation>
+											The use_sliding_expiration value is whether you wish to use a sliding expiration or not. Defaults
+											to false. Not all providers support this setting, it may be ignored. Check their respective
+											documentation.
+										</xs:documentation>
+									</xs:annotation>
+								</xs:enumeration>
 								<xs:enumeration value="query.substitutions" />
 								<xs:enumeration value="query.factory_class" />
 								<xs:enumeration value="query.linq_provider_class" />