Ensure units do get in attacking range since that range can change over time because of elevation differences.
This is a partial revert of rP22351, which skipped the "MoveToTargetAttackingRange" in APPROACHING. I (incorrectly) assumed that the original order was still perfectly fine, but in fact the attacker's max range may have changed as that depends on the relative elevation between attack and target - and so the original order might never get us in range!
This was introduced originally in rP13626.
Add a comment to clarify this.
Further, this makes sure UnitMotion still is aware that it has a target even if it is in range from the beginning, as that could lead
to stuckiness (and did when chasing sometimes). This was done in D1984 anyways.
Differential Revision: https://code.wildfiregames.com/D2035