Rewrite the "cluster" section of the rewrite docs. Changes;

    * Call the technique "sharding" - like the rest of the world does. 

    * Simplify to just one example use-case, sharding users.

    * Make the usage of the term "physical host" consistent.

    * Remove unreachable nonsense rewrite rules

    * Remove redundant sentences.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@832261 13f79535-47bb-0310-9956-ffa450edef68