[O] EXPORT_FILENAME Doesn't Seem to Be Documented.

[Ian Barton]

Following my earlier post about how to specify and export filename on a 
per buffer basis the variable EXPORT_FILENAME doesn't seem to be 
documented in anywhere in the current git version of org, although it 
appears in http://orgmode.org/manual/Export-settings.html.

I think it should appear at least in "Summary of in-buffer settings" as 
well as "Export Settings". I am happy to provide a patch, but before I 
do is there anywhere else it should be documented?

Ian.