SPARK-5744 [CORE] Take 2. RDD.isEmpty / take fails for (empty) RDD of Nothing

Follow-on to https://github.com/apache/spark/pull/4591

Document isEmpty / take / parallelize and their interaction with (an empty) RDD[Nothing] and RDD[Null]. Also, fix a marginally related minor issue with histogram() and EmptyRDD.

CC rxin since you reviewed the last one although I imagine this is an uncontroversial resolution.

Author: Sean Owen <sowen@cloudera.com>

Closes #4698 from srowen/SPARK-5744.2 and squashes the following commits:

9b2a811 [Sean Owen] 2 extra javadoc fixes
d1b9fba [Sean Owen] Document isEmpty / take / parallelize and their interaction with (an empty) RDD[Nothing] and RDD[Null]. Also, fix a marginally related minor issue with histogram() and EmptyRDD.