Document list of functions that are safe to call in "pending exception" state

[RReverser]

#122 introduced a bunch of functions that can be in fact called when an exception is pending - this is important for cleanups on the consumer side in case of a failure.

As discussed, it's useful to make at least a conservative list of functions that consumers could rely on to be safe to call in a pending exception state across all the engines for purposes of cleanup.