Document a requirement of caching with composite id.

Author: fredericDelaporte

doc/reference/modules/component_mapping.xml

@@ -225,6 +225,12 @@
                     notion of composite key equality.
                 </para>
             </listitem>
+            <listitem>
+                <para>
+                    It should re-implement <literal>ToString()</literal> if you consider
+                    using the second level cache. See <xref linkend="NHibernate.Caches-howto" />.
+                </para>
+            </listitem>
         </itemizedlist>
 
         <para>

doc/reference/modules/nhibernate_caches.xml

@@ -166,6 +166,12 @@
 				transactions. Interacting with the data store without an explicit transaction is discouraged, and will
 				not allow the second level cache to work as intended.
 			</para>
+			<para>
+				To avoid issues with composite ids and some cache providers, <literal>ToString()</literal> needs to
+				be overridden on composite id classes. It should yield an unique string representing the id. If the
+				composite id is mapped as a component, overriding the component <literal>ToString()</literal>
+				is enough. See <xref linkend="components-compositeid" />.
+			</para>
 		</formalpara>
 		<para>
 			See also <xref linkend="performance-cache" />.