* Call FailureSignalHandlerOptions.writenfn with nullptr at the end

This behavior has already been alluded to in the document, but it hasn't been
implemented.

This PR calls changes the failure signal handler to call writerfn(nullptr) at
the end of the message block. It also clarifies the documentation.

https://github.com/abseil/abseil-cpp/issues/933

* Update failure_signal_handler.h

Co-authored-by: Derek Mauro <761129+derekmauro@users.noreply.github.com>