Document current limitation of rdd.fold.

This does not resolve SPARK-6416 but just documents the issue.
CC JoshRosen

Author: Sean Owen <sowen@cloudera.com>

Closes #6231 from srowen/SPARK-6416 and squashes the following commits:

9fef39f [Sean Owen] Add comment to other languages; reword to highlight the difference from non-distributed collections and to not suggest it is a bug that is to be fixed
da40d84 [Sean Owen] Document current limitation of rdd.fold.