SPARK-1117: update accumulator docs

The current doc hints spark doesn't support accumulators of type `Long`, which is wrong.

JIRA: https://spark-project.atlassian.net/browse/SPARK-1117

Author: Xiangrui Meng <meng@databricks.com>

Closes #631 from mengxr/acc and squashes the following commits:

45ecd25 [Xiangrui Meng] update accumulator docs
(cherry picked from commit aaec7d4a80ed370847671e9e29ce2e92f1cff2c7)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>