Add doc for "coldStartStrategy" usage in ALS

SPARK-14489 adds the ability to skip NaN predictions during ALS.transform. This can be useful in production scenarios but is particularly useful when trying to use the cross-validation classes with ALS, since in many cases the test set will have users/items that are not in the training set, leading to evaluation metrics that are all NaN and making cross-validation unusable.

Add an explanation for the coldStartStrategy param to the ALS documentation, and add example code to illustrate the usage.