Update the "how to configure HDFS" documentation to include new features

There has been lots of cases where HDFS administrators have enquired about setup and configuration of HDFS. A recent question asked about configuring Datanodes to understand the rack that it belongs to. There is a wiki page that is out-of-date:

http://wiki.apache.org/lucene-hadoop/HowToConfigure

Some new things that come to mind are:
1. rack location for Datanodes.
2. Trash configuration
3. SecondaryNamenode config parameters