Correct missing javadoc comments in java.rmi module

Summary

Address the "no comment" warnings in java.rmi generated by javadoc -Xdoclint

Problem

java.rmi contains several classes such as java.rmi.server.RemoteObject and java.rmi.server.UnicastRemoteObject containing fields and/or methods which are missing a javadoc comment.

Solution

Add the missing comments to the appropriate fields and methods in java.lang and java.io. Also added java.io.Serial annotation on serialization related methods and fields.

Specification

See the attached diff for changes to:

java/rmi/activation/ActivationGroupDesc.java
java/rmi/activation/ActivationID.java
java/rmi/server/RemoteObject.java
java/rmi/server/UnicastRemoteObject.java