Update doc for VEHICLE_SPEED_DISPLAY_UNITS.

Part of a large doc update. Add doc for new supported value range
APIs. For be backward compatible with older VHAL clients still
using deprecated fields, we still require the old fields to be
populated.

Test: None
Bug: 371636116
Flag: EXEMPT no code change
Change-Id: Ib965ef400b00c1ccc719862c2dcd6bae16da0cc2