Update AE_MODE_ON description for flash control

This change updates the behavior for manual flash strength control when
AE_MODE is ON. If the device supports manual flash strength 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: I1ecb73a643f79330f92031a338dce13e7fe752f2