CXX-283 Clarify documentation for startRunningPeriodicTasks

The old documentation stated that startRunningPeriodicTasks() could be called multiple times.  This isn't true, doing so will lead to [an assertion](https://github.com/mongodb/mongo-cxx-driver/blob/legacy/src/mongo/util/background.cpp#L185-L187).

Now that initialize() has been clarified users should not be able to cause startRunningPeriodicTasks() to be called twice, but I think it's worth fixing the documentation.