[Docs] Add some release notes for X86 target.
[Docs] Add some release notes for X86 target.

git-svn-id: ee41f7a47e00113e4a3f1d4cbf84f6e482c3caa7@308831 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm