[xsl] where did doc-available() come from?

[Mark Giffin <m1879@xxxxxxxxxxxxx>]

In my 2004 version of XPath 2.0 Programmer's Reference by Kay, which 
I've been using for many years, there is no doc-available() function. I 
have recently seen mention of this function in forums, and it sounded 
very useful, but it confused me because I'd never heard of it. But in my 
Kindle version of the same book copyrighted 2008, it has 
doc-available(). Was this added to the spec between '04 and '08? I've 
been missing out.

Mark