Add reference for appoximate unique algorithms in java and python SDK

The java [1] and python [2] classes for ApproximateUnique should reference a paper or some other form of online documentation as a source for their algorithm. 

 [1] https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/ApproximateUnique.java

[2] https://github.com/apache/beam/blob/master/sdks/python/apache_beam/transforms/stats.py