[FSSDK-12670] Add comment explaining the < 2 identifiers guard

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jaeopt

core-api/src/main/java/com/optimizely/ab/odp/ODPEventManager.java

@@ -151,6 +151,8 @@ public void identifyUser(@Nonnull Map<String, String> identifiers) {
             }
         }
 
+        // An identify event requires at least 2 identifiers to link (e.g., vuid + fs_user_id).
+        // A single identifier has no cross-reference value and would generate unnecessary traffic.
         if (validIdentifiers.size() < 2) {
             logger.debug("ODP identify event is not dispatched (fewer than 2 valid identifiers).");
             return;