Custom XPath functions (Action 2009-10-28.4)

All,

I did a pass at enhancing the "Custom XPath functions" page [1].

A few comments:

* It seemed appropriate to borrow text from the XSLT 2.0 specification.
* It doesn't seem like we can just use the XSLT 2.0 text however (be it only
because we need to support XPath 1.0).
* I also borrowed the <value-of> and a subset of <sequence> from XSLT 2.0.
To be discussed!
* This is not spec-ready text, and there is still a bunch of TODO items left
to address.

Hopefully we can discuss all of this next week at the f2f!

This closes action 2009-10-28.4.

-Erik

[1] http://www.w3.org/MarkUp/Forms/wiki/Custom_XPath_functions