Improve documentation around transaction and locking

As suggested by Jonathan Halliday:

• preserving some subset of the existing transactional guarantees is all well and good BUT if the user is relying on additional 'guarantees' that are not preserved in all cases then they'll be in trouble. Therefore, it's essential to document not just what the minimum expected guarantees for a given config are, but also that no additional properties that may coincidently be observed are actually guaranteed. Some vendors go further and explicitly document the bad things that may happen with given settings, since in some cases these are not easy to reproduce in testing.