Motors: document behaviour of VAL

As a developer, I would like to have documentation about how VAL on a motor reacts in the following situations:
- Motor moves and reaches it's setpoint normally (does VAL get reset to match the final readback or not?)
- Motor is explicitly stopped during a move (I believe VAL gets set to match RBV)
- Motor stops due to hitting a physical limit
- Motors stalls / abnormally terminates a move
- An external move larger than encoder deadband (e.g. noisy encoder, jog box)
- The IOC is restarted (e.g. config change)

Plus any other interesting cases we can think of.

Once we have documented the above, we should think about whether that behaviour is correct or not. If not, create further tickets to change the behaviour as appropriate (those changes are not part of this ticket)

**Acceptance criteria**
- There is documentation on the wiki about how VAL reacts in each of the situations listed above
- For each case we have considered whether that is the correct behaviour or not.
- If these behaviours are implemented in the driver as opposed to the motor record, we have checked that the behaviours are consistent for at least McLennans, Galils and Beckhoffs.