[SPARK-3136][MLLIB] Create Java-friendly methods in RandomRDDs

Though we don't use default argument for methods in RandomRDDs, it is still not easy for Java users to use because the output type is either `RDD[Double]` or `RDD[Vector]`. Java users should expect `JavaDoubleRDD` and `JavaRDD[Vector]`, respectively. We should create dedicated methods for Java users, and allow default arguments in Scala methods in RandomRDDs, to make life easier for both Java and Scala users. This PR also contains documentation for random data generation. @brkyvz
I marked `RandomDataGenerator` as a developer API and didn't mention it in the guide.