Correct add call during video call config docs

Update the carrier config docs for
KEY_ALLOW_ADD_CALL_DURING_VIDEO_CALL_BOOL. Adding calls during a video
call is only allowed if the carrier config is set to true.

Bug: 396638090
Flag: EXEMPT docs only
Test: build
Change-Id: I3399d7d239ab8984fa96e1951329a6ceb6668f15