Add note about resetting `times_considered`
Resetting `times_considered` after multiple
serializations of a particular transition are performed
to give the checker all exections of an actor
that can perform multiple actions. There was concern
that this value would have to be "reset" to the
original value after the serialization, since the latter
modifies the simcall's observer's internal state.
However, no reset is actually needed as each
SIMCALL_EXECUTE message that the checker sends to the
application-side comes equipped with the latest
`times_considered`, and thus preparation will be
performed correctly before execution.
