和mapValues相似，但是将map函数返回的value进行展开并和key一起返回。
函数原型：
def flatMapValuesU: RDD[(K, U)]
例子：
val a = sc.parallelize(List("dog", "tiger", "lion", "cat", "panther", "eagle"), 2)
val b = a.map(x => (x.length, x))
b.flatMapValues("x" + _ + "x").collect
res6: Array[(Int, Char)] = Array((3,x), (3,d), (3,o), (3,g), (3,x), (5,x), (5,t), (5,i), (5,g), (5,e), (5,r), (5,x), (4,x), (4,l), (4,i), (4,o), (4,n), (4,x), (3,x), (3,c), (3,a), (3,t), (3,x), (7,x), (7,p), (7,a), (7,n), (7,t), (7,h), (7,e), (7,r), (7,x), (5,x), (5,e), (5,a), (5,g), (5,l), (5,e), (5,x))