[ReleaseNotes] Added bullet point stating that APFloat::isNormal() is now IEEE 754R-2008 compliant and that the relevant method renaming occurred.

For more information see r184449, r184350, r184356, r184366.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184452 91177308-0d34-0410-b5e6-96231b3b80d8