Update doc for SEAT_HEADREST_FORE_AFT

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: 383184445
Flag: EXEMPT no code change
Change-Id: I10113c78f628d5f07eb93682809ecb4a464e2d7a