add doc for FPGrowth.train()

Author: yanboliang

python/pyspark/mllib/fpm.py

@@ -55,6 +55,13 @@ class FPGrowth(object):
 
     @classmethod
     def train(cls, data, minSupport=0.3, numPartitions=-1):
+        """
+        Computes an FP-Growth model that contains frequent itemsets.
+        :param data:            The input data set, each element contains a transaction.
+        :param minSupport:      The minimal support level (default: `0.3`).
+        :param numPartitions:   The number of partitions used by parallel FP-growth
+                                (default: same as input data).
+        """
         model = callMLlibFunc("trainFPGrowthModel", data, float(minSupport), int(numPartitions))
         return FPGrowthModel(model)