[Docs] Update cache()'s storage level to be consistent with the code

[wjur]

The docs say that the default storage level is MEMORY_ONLY,
but in fact it is MEMORY_AND_DISK. This commit fixes the
inconsistency between the code and the docs.

[wjur]

The issue is also valid for 1.6 (not only for the master branch).

Please refer to
https://github.com/wjur/spark/blob/wjur/caching_default_level/sql/core/src/main/scala/org/apache/spark/sql/execution/CacheManager.scala#L84
and to
https://github.com/wjur/spark/blob/wjur/caching_default_level/sql/core/src/main/scala/org/apache/spark/sql/DataFrame.scala#L1601

[AmplabJenkins]

Can one of the admins verify this patch?

[wjur]

@mateiz, I'm not sure who is responsible for the docs, but it seems that you are the last one who edited the affected lines. I would be grateful if you could take a look at the PR. The inconsistency was introduced by PR #2686 (if that matters).

[srowen]

I don't think that's correct:

  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
  def persist(): this.type = persist(StorageLevel.MEMORY_ONLY)

  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
  def cache(): this.type = persist()

[wjur]

That's confusing... @srowen, you're correct. RDDs are not DataFrames. I'm really sorry for taking your time.