[docs] Describe how _alwaysEmitIntoClient relates to library evolution.
[docs] Describe how _alwaysEmitIntoClient relates to library evolution.

JGiola/swift,benlangmuir/swift,ahoppen/swift,atrick/swift,roambotics/swift,gregomni/swift,ahoppen/swift,benlangmuir/swift,gregomni/swift,atrick/swift,atrick/swift,ahoppen/swift,benlangmuir/swift,atrick/swift,xwu/swift,apple/swift,benlangmuir/swift,rudkx/swift,gregomni/swift,roambotics/swift,rudkx/swift,apple/swift,roambotics/swift,JGiola/swift,JGiola/swift,benlangmuir/swift,ahoppen/swift,xwu/swift,xwu/swift,apple/swift,roambotics/swift,JGiola/swift,JGiola/swift,rudkx/swift,glessard/swift,rudkx/swift,xwu/swift,apple/swift,gregomni/swift,apple/swift,roambotics/swift,gregomni/swift,glessard/swift,xwu/swift,xwu/swift,roambotics/swift,glessard/swift,ahoppen/swift,rudkx/swift,atrick/swift,xwu/swift,JGiola/swift,benlangmuir/swift,rudkx/swift,ahoppen/swift,glessard/swift,gregomni/swift,atrick/swift,apple/swift,glessard/swift,glessard/swift