[SPARK-3356] [DOCS] Document when RDD elements' ordering within partitions is nondeterministic

As suggested by @mateiz , and because it came up on the mailing list again last week, this attempts to document that ordering of elements is not guaranteed across RDD evaluations in groupBy, zip, and partition-wise RDD methods. Suggestions welcome about the wording, or other methods that need a note.
I don't think this is a good place to put this in the programming guide. If you can't find another place, maybe just leave it out. The other note here is a much more important and more common pitfall.