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.