Update AE_MODE_ON description for flash control.

This change updated the behavior for manual flash strength control when
AE_MODE is ON. If the device supports manual flashs trength control and
AE_MODE is ON, ae_precapture trigger should be fired in order to
determine the settings at strength level during the still capture in
order to avoid the image being incorrectly exposed.

Bug: 368182722
FLAG: EXEMPT, doc update
Change-Id: I0cd72477088df9022ec86894fe32cc73f3d48cbe