It's documented. I used to be very fluent in the AS3LR, and I found it in a minute or two here: http://help.adobe.com/en_US/FlashPlatform/reference/actionsc...

The formal language specification came fairly late in AS3's useful life, but it also describes the syntax: http://help.adobe.com/livedocs/specs/actionscript/3/as3_spec...