[Misc] Clarify the reason for a single cache even though there can be multiple config wiki pages

xwiki-platform-core/xwiki-platform-mail/xwiki-platform-mail-send/xwiki-platform-mail-send-default/src/main/java/org/xwiki/mail/internal/configuration/MainWikiSendMailConfigClassDocumentConfigurationSource.java

@@ -43,6 +43,9 @@ public class MainWikiSendMailConfigClassDocumentConfigurationSource
     @Override
     protected String getCacheId()
     {
+        // Note: we use a single cache id because this module is installed on the root namespace and thus there's
+        // only a single MainWikiSendMailConfigClassDocumentConfigurationSource component in the farm (and thus a single
+        // cache).
         return "configuration.document.mail.send.mainwiki";
     }
 

xwiki-platform-core/xwiki-platform-mail/xwiki-platform-mail-send/xwiki-platform-mail-send-default/src/main/java/org/xwiki/mail/internal/configuration/SendMailConfigClassDocumentConfigurationSource.java

@@ -42,6 +42,9 @@ public class SendMailConfigClassDocumentConfigurationSource
     @Override
     protected String getCacheId()
     {
+        // Note: we use a single cache id because this module is installed on the root namespace and thus there's
+        // only a single SendMailConfigClassDocumentConfigurationSource component in the farm (and thus a single
+        // cache).
         return "configuration.document.mail.send";
     }