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

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3107 from sryza/sandy-spark-4230 and squashes the following commits:

37a1d19 [Sandy Ryza] Clear up a couple things
34d53de [Sandy Ryza] SPARK-4230. Doc for spark.default.parallelism is incorrect

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