[SPARK-18353][CORE] spark.rpc.askTimeout defalut value is not 120s

## What changes were proposed in this pull request?

Avoid hard-coding spark.rpc.askTimeout to non-default in Client; fix doc about spark.rpc.askTimeout default

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #15833 from srowen/SPARK-18353.