JDK 14 RFR of JDK-8230681: Add @since tag to java.io.Serial

[Joe Darcy]

Hello,

As spotted by Martin, please review the addition of an @since tag to the 
new-in-JDK-14 java.io.Serial type; patch below.

Thanks,

-Joe

diff -r 06f3d5092832 src/java.base/share/classes/java/io/Serial.java
--- a/src/java.base/share/classes/java/io/Serial.java    Thu Sep 05 
11:12:12 2019 -0700
+++ b/src/java.base/share/classes/java/io/Serial.java    Thu Sep 05 
15:24:42 2019 -0700
@@ -95,6 +95,7 @@
  *
  * @see Serializable
  * @see Externalizable
+ * @since 14
  */
 @Target({ElementType.METHOD, ElementType.FIELD})
 @Retention(RetentionPolicy.SOURCE)