The behaviour described above was removed in
812a4b412a, thus henceforth
contradicting `RevertResult`'s xmldoc. As it is relied on by some
external rulesets, bring it back to unbreak them.
I also took the freedom to add type checking, as we can't limit the
usage of `Add()` since it's a Container. The exception thrown also
advises of using the suggested `AddTrigger()` instead.
Functionally equivalent right now, but the combined variant is more
localised to what it actually needs to do, and less error-prone if
any new code gets appended to the method.