Camera: Document behavior when cropRegion and zoomRation conflict

In case cropRegion == windowboxing and zoomRatio != 1.0f, specify the
camera framework behavior of overriding cropRegion.

Test: CaptureRequestTest#testZoomRatio
Bug: 172873869
Change-Id: I9f3f51bca431841feacb4b2cce702dea179bacb9