Document Dom's general assumptions

There are a couple of general guidance rules we have settled on as part of doing Hyperion that we should document:

* Plan steps should not tidy up after themselves - this aides in allowing speed ups, the obvious example where this causes issues is where GDA will put backlights back into the beam after data collections even if the next operation is another data collection.
* Not checking if you are already in position when doing moves

## Acceptance Criteria
- The above is documented somewhere