If you move the Terminal split bar to the left (this will widen the Terminal window), the icon for Move to Bottom will appear.

Updated on 9-May-2022:
The current VSCode version as now is 1.67.0. The below steps are still valid in current version of UI.
Updated on 18-Nov-2018:
Version 1.29 was released in October 2018 (https://code.visualstudio.com/updates/v1_29) and the UI has changed for configuring the terminal position.
1. Version check

2. Move Terminal From Right To Bottom

3. Move Terminal From Bottom To Right
