[OpenCL][Doc] Added OpenCL vendor extension description to user manual doc
[OpenCL][Doc] Added OpenCL vendor extension description to user manual doc

Added description of a new feature that allows to specify
vendor extension in flexible way using compiler pragma instead
of modifying source code directly (committed in clang@r289979).

Review: D29829



git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@295313 91177308-0d34-0410-b5e6-96231b3b80d8

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