Documentation on transaction priorities

The application developer can setup priority on transactions. These priorities have huge effect on how these transactions perform and fail, which is the desired effect.

Especially batch transactions can be suspended entirely if the cluster is under load. For example, data movement caused by host failure. In some cases, this can be suspended very long as the data movement lasts 10s of minutes.

We have some documentation on API docs and transaction options. We should make this more explicit.

Start with the existing API documentation and transaction options documentation mentioned in the issue, then review how transaction priorities affect batch transactions during cluster load. Done means the documentation explicitly explains the priority behavior and the possibility of prolonged suspension during data movement.

Written by the indexing model from the issue text.