Well, I don't think that inserting something into source code is always an option... so it should be probably understood as in situations a, b, c and d the semicolon is optional, otherwise it is mandatory. So, practically, it says that when you cannot parse something, but you are not sure it's not an error, imagine there was a semicolon after the last line break and try again, but I don't think there may be a generic way to express this in BNF.
Also note that given AS3 metadata extension there are new cases to handle: [Bindable]; public var foo:int; note the semicolon after the metatag - it will render it as an array literal with Bindable as an expression.
