Add some basic documentation for the __attribute__((target(""))) support.
Add some basic documentation for the __attribute__((target(""))) support.

Describes the general syntax of how it's used with the unfortunate
usage of "subtarget features" and some examples from the x86 port
to help users.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@241524 91177308-0d34-0410-b5e6-96231b3b80d8

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