SPARK-4230. Doc for spark.default.parallelism is incorrect

My thinking was that Spark's APIs have no mention of the concept of a "shuffle partition" (e.g. the term is referenced nowhere on https://spark.apache.org/docs/latest/programming-guide.html), but even novice Spark users are meant to understand that every transformation has input and output RDDs and that every RDD has a number of partitions.

Maybe "Default number of partitions for the RDDs produced by operations like ..."?