Update clang attribute document.
Update clang attribute document.

Remove the instructions on manually adding boilerplate code of AttributeList::Kind and AttributeList::getKind().
Both are now generated by tblgen.



git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@158386 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang