[XQuery] IBM-XQ-013: Delete unnecessary note

(IBM-XQ-013) Section 4.12 (Function Declaration): The Note at the very end 
of this section should be deleted. There is no need for the XQuery 
document to speculate about future versions of the language, especially 
since, if properly designed, these hypothetical future versions need not 
introduce any incompatabilities.

--Don Chamberlin