[F&O] IBM-FO-009: Fixes to namespaces and prefixes

(IBM-FO-009) Section 1.7, Namespaces and Prefixes, needs the following 
changes: 

(a) Section claims that all functions defined in this document are in 
three namespaces. Actually it should be four (xdt: contains some 
constructor functions).

(b) The first paragraph and the last paragraph both say that fn: functions 
are visible to users and op: functions are not. There's no need to say 
this twice.

(c) In the bullet list, it would be helpful to list the pre-assigned 
prefixes as well as the namespace URIs (as in the XQuery book, section 
4.4).

--Don Chamberlin