Add missing package-info files for common packages

Issue: SPR-14420

spring-tx/src/main/java/org/springframework/transaction/annotation/package-info.java

@@ -1,5 +1,5 @@
 /**
- * Java 5 annotation for transaction demarcation.
+ * Spring's support for annotation-based transaction demarcation.
  * Hooked into Spring's transaction interception infrastructure
  * via a special TransactionAttributeSource implementation.
  */

spring-tx/src/main/java/org/springframework/transaction/event/package-info.java

@@ -0,0 +1,4 @@
+/**
+ * Spring's support for listening to transaction events.
+ */
+package org.springframework.transaction.event;