Reconsider and document behavior of Xt.post_commit

[polytypic]

Currently, if an action registered with Xt.post_commit raises an exception, then actions registered later will not be called. At the very least this and the order in which actions will be called should be documented, but it might make sense to reconsider the order in which the actions are called and whether it would make sense to call all of the actions even in case one or more actions raise exception.