[Docs] Document the @_spiOnly attribute
[Docs] Document the @_spiOnly attribute

roambotics/swift,apple/swift,glessard/swift,glessard/swift,roambotics/swift,roambotics/swift,roambotics/swift,apple/swift,apple/swift,glessard/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,glessard/swift,glessard/swift,apple/swift,benlangmuir/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,apple/swift