Improve the single node setup docs for MR

I'm gonna be recording all nits I can about the page at http://hadoop.apache.org/common/docs/r0.23.0/hadoop-yarn/hadoop-yarn-site/SingleCluster.html here:

• The section at http://hadoop.apache.org/common/docs/r0.23.0/hadoop-yarn/hadoop-yarn-site/SingleCluster.html#Mapreduce_Tarball is suitable to developers, not users. We should instead just point to releases page for tarball.
• On http://hadoop.apache.org/common/docs/r0.23.0/hadoop-yarn/hadoop-yarn-site/SingleCluster.html#Setting_up_the_environment since the tarball structure is definite, we could do better to either point to a link to configure HDFS and Common or provide instructions ourselves. Same applies to path the MR envs ought to be set to – none of these are mentioned.
• Under http://hadoop.apache.org/common/docs/r0.23.0/hadoop-yarn/hadoop-yarn-site/SingleCluster.html#Setting_up_Configuration. one of the "Add the following configs" text is defined as a heading, while other isn't.
• The "Create Symlinks" and "Running daemons" sections do not appear in the index on top.
• Remove the "Good luck." at the end – Users shouldn't need luck to setup good stuff

OT comment: The "Create Symlinks" part is curious. That we need symlinks to run MR looks very wrong to me.