[boost] [statechart] Documentation update suggestion.

Hi. Just an idea: update the documentation to explicitly note that statechart using code should synchronize both processEvent() and terminate() calls when using a single statechart from multiple threads. We just solved a bug in one of our projects using the statechart library where we forgot to synchronize the terminate() call. Was causing non-reproducible assertion failures in the termination code. :-) Hope this helps. Best regards, Jurko Gospodnetić