TRANS13 — OTS register_resource clarification

Summary: I think it might be useful to add some clarification text to
register_resource in the OTS specification. If the invoked Coordinator
represents a subtransaction, and the resource is not a
SubtransactionAwareResource, then the resource is registered as a
participant with the current transaction, but will only receive
commit/rollback requests when the top-level ancestor terminates. The
implication is that if the subtransaction rolls back it has no effect on
resources registered in this way, i..e, they remain registered with the
top-level transaction.

See Proposed Resolution