Audio: Document channel order convention

This patch restores the documentation on the channel order convention
that was present in the Audio HAL 2.0 but remove by mistake in 4.0.

This is a vendor feedback.

Test: mm
Bug: 133453897
Change-Id: I8eabd8883612d39ced21481fc44661b0808754bb
Signed-off-by: Kevin Rocard <krocard@google.com>