Add note to changelog about Duration.split.

Martin asked me to add an entry to the changelog about the new `Duration.split`, so here it is.
It looks like CXREF is defined in std.ddoc, and changelog.html is built without including that:

    changelog.html : $(DDOC) changelog.dd
    DDOC=macros.ddoc windows.ddoc doc.ddoc $(NODATETIME)

I'm building with `make -f win32.mak changelog.html`, and it's undefined. Do the site docs get built differently?