The autodocs for ReAction classes are contained in the OS3.5 NDK (as found on the developer CD) and the OS3.9 NDK (found where the Boingbags can be downloaded from).
Other than that there is no SDK for ReAction. You can look at the examlpes in the ClassAct2Demo archive on Aminet, just change every macro beginning with CA_ into RA_, then they compile fine IIRC.
The ClassAct SDK was available to registered users of ClassAct only. I don't know if ClassAct can still be registered, though, and it costs money of course.
Bye,
Thomas