Clarify WriteOperations' ensureParentsViaMerge parameter

Ensuring parents is a relatively expensive operation, which is not sufficiently documented in the API contract and design. The users are lead to believe the two options have roughly equal cost, which is certainly not true.

Moreover, this operation is only meaningful when the InstanceIdentifier crosses either a presence container or a list entry - otherwise it behaves exactly the same, except it has a higher overhead.

Document this behaviour and express it in API design by splitting off the "true" operation, so as to increase documentation visibility as well as making call sites more visible.