The only documentation I know of is from the Adobe LiveDoc on Flash (used to be Flash 8, now it's in the CS3 section, it's the same document though).
The direct link is
http://livedocs.adobe.com/flash/9.0/mai ... 00344.html
If this doesn't work, go to the LiveDoc root and search for "External API XML format", sooner or later a hit will not be on JavaScript communication but lead you there.