More progress on ACTION-1820 (valid function)

All,

Here is the latest version. I have integrated the group's feedback on
using *should* for dispatching
xforms-binding-exception/xforms-compute-exception, and adding
precisions on what could go wrong when using valid() on binds.

http://www.w3.org/MarkUp/Forms/wiki/XPath_Expressions_Module#The_valid.28.29_Function

-Erik