Add javadoc

Author: trask

instrumentation/java-http-client/library/src/main/java/io/opentelemetry/instrumentation/httpclient/internal/Experimental.java

@@ -13,6 +13,9 @@
  * This class is internal and experimental. Its APIs are unstable and can change at any time. Its
  * APIs (or a version of them) may be promoted to the public stable API in the future, but no
  * guarantees are made.
+ *
+ * @deprecated Use {@link io.opentelemetry.instrumentation.javahttpclient.internal.Experimental}
+ *     instead.
  */
 @Deprecated
 public final class Experimental {