[XQuery] IBM-XQ-012: Default function namespace

(IBM-XQ-012) Section 4.5 (Default Namespace Declaration) says that the 
default function namespace can be set to the empty string, indicating no 
namespace.  A note should be added, stating that if this is done, every 
function call must be have an explicit namespace prefix, since every 
function is required to be in a namespace.

--Don Chamberlin