The new expressions will be added to the manual once a stable release is out (in about one or two weeks).
I can't tell for sure, but apparently the number between parenthesis could be referring to the touch "number" in a multitouch. (you could have 3 simultaneous touches with different angle, the index allows you to refer to the touch you want. 0 is the first/default one)
I guess if you test on desktop with the mouse input set to yes, you would only refer to AngleAt(0).A quick meaningless example
Apparently AngleAt() automatically detects the current angle of the touch depending on the position where the touch started.