Format the SetCallBackKey description more nicely

Go item by item, and reword things a bit.

Change-Id: Ie16d034214cc721a465c8822edc6e3c6c759cd9b
Roll changelog

Change-Id: Ibb1becc4400221feebe586e6de482fefd555f565
Make draft-wilkinson-afs3-rxgk-afs-03

Change-Id: I84dfe167940dc86f02f6e680af73703fc6d639f3