ACTION-2025 - Deal with the naming over xpath versions

All,

Here is an attempt at clarifying the function namespaces with a table:

https://www.w3.org/MarkUp/Forms/wiki/XPath_Expressions_Module#The_XForms_Function_Library

I also:

- separated the part about error handling, which was awkward
- removed the reference to following sections, which seemed unneeded

And here is the diff:


https://www.w3.org/MarkUp/Forms/wiki/index.php?title=XPath_Expressions_Module&diff=4209&oldid=4200

One question is whether we call "XForms Function Library" only the
functions which are *added* by XForms, or the entire function library,
including the standard XPath (1.0 or 2.0) functions. For now I have assumed
that "XForms Function Library" only contains what is added by XForms.

-Erik