[SPARK-2683] unidoc failed because org.apache.spark.util.CallSite uses Java keywords as value names

Renaming `short` to `shortForm` and `long` to `longForm`.

JIRA: https://issues.apache.org/jira/browse/SPARK-2683

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1585 from yhuai/SPARK-2683 and squashes the following commits:

5ddb843 [Yin Huai] "short" and "long" are Java keyworks. In order to generate javadoc, renaming "short" to "shortForm" and "long" to "longForm".