[SPARK-4361][Doc] Add more docs for Hadoop Configuration

I'm trying to point out reusing a Configuration in these APIs is dangerous. Any better idea?
In fact, there are many uses of `sc.hadoopConfiguration` in the wild which assume that it's shared: 

https://github.com/search?q=sc.hadoopConfiguration&type=Code&utf8=%E2%9C%93

Most of those are using it to configure S3 credentials.