[XQuery] IBM-XQ-022: Casting QName to string

(IBM-XQ-022) The following parts of the XQuery document depend on the 
ability to cast any atomic value into a string:
(a) Section 3.7.1.1, Direct Element Constructors--Attributes, Rule 3b.
(b) Section 3.7.1.3, Direct Element Constructors--Content, Rule 1d.
(c) Section 3.7.3.1, Computed Element Constructors, Content Expression 
Rule 1.
At present, the Functions and Operators document does not permit a QName 
to be cast into a string. It is clearly not acceptable to be unable to 
construct any element or attribute that contains a QName. This 
inconsistency between the XQuery and Functions and Operators documents 
needs to be corrected.

Note that casting a QName into a string is also required by the 
Serialization document, as noted in comment IBM-SE-015.

--Don Chamberlin