8305206: Add @spec tags in java.base/java.* (part 1)

Reviewed-by: alanb, naoto, darcy, lancea, dfuchs, iris, mchung

src/java.base/share/classes/java/io/Serial.java

@@ -94,6 +94,7 @@ import java.lang.annotation.*;
  * and methods reflectively and those fields and methods may appear
  * otherwise unused in a {@code Serializable} class.
  *
+ * @spec serialization/index.html Java Object Serialization Specification
  * @see Serializable
  * @see Externalizable
  * @since 14