Follow-up 15a2781: Add in-code comment on alias for when it was added

Change-Id: I7fd16236c60cb5e1f2d09b9bdb09df130d345c71

RELEASE-NOTES-1.41

@@ -627,6 +627,7 @@ because of Phabricator reports.
   - SiteStatsInit -> MediaWiki\SiteStats\SiteStatsInit
   - MediaWiki\ExtensionInfo -> MediaWiki\Utils\ExtensionInfo
   - GitInfo -> MediaWiki\Utils\GitInfo
+  - MWTimestamp -> MediaWiki\Utils\MWTimestamp
 * The following methods, previously deprecated, now emit deprecation warnings:
   - ConfigFactory::getDefaultInstance(), deprecated since 1.27
   - AbstractContent::getNativeData(), deprecated since 1.33

includes/utils/MWTimestamp.php

@@ -156,4 +156,7 @@ class MWTimestamp extends ConvertibleTimestamp {
 	}
 }
 
+/**
+ * @deprecated since 1.41
+ */
 class_alias( MWTimestamp::class, 'MWTimestamp' );