CXX-303 clarify initialize() behavior

-Implementation was changed to only allow initialize() to be called once, and to allow shutdown() to only be called once after a call to initialize().
I think maybe your atomic should go as follows:
0 (static initialization): 'initialize has not been called'. Legal actions: call initialize.
1 (set by client::initialize running OK): 'initialize has been called'. Legal actions: call terminate
-1 (set by client::terminate running OK): 'terminate has been called'. Legal actions: none.