Document new COLLATION_KEY function

Please add a small entry to phoenix.csv (which lives in https://svn.apache.org/repos/asf/phoenix) to describe how to use the new COLLATION_KEY built-in function. You can copy/paste an existing function description and see examples here: https://phoenix.apache.org/language/functions.html. For directions on updating the website, see here: https://phoenix.apache.org/language/functions.html