update the big red warning at the top. Most of the old content remains
and awaits revision.
Clear out the API changes section, and start it up again with a
mention of the add->fadd transition.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102977 91177308-0d34-0410-b5e6-96231b3b80d8