[SPARK-6490][Docs] Add docs for rpc configurations

Added docs for rpc configurations and also fixed two places that should have been fixed in #5595.

Author: zsxwing <zsxwing@gmail.com>

Closes #5607 from zsxwing/SPARK-6490-docs and squashes the following commits:

25a6736 [zsxwing] Increase the default timeout to 120s
6e37c30 [zsxwing] Update docs
5577540 [zsxwing] Use spark.network.timeout as the default timeout if it presents
4f07174 [zsxwing] Fix unit tests
1c2cf26 [zsxwing] Add docs for rpc configurations