[Docs] Expand -fstack-protector and -fstack-protector-all
[Docs] Expand -fstack-protector and -fstack-protector-all

Improve the description of these command line options
by providing specific heuristic information, as outlined
for the ssp function attribute(s) in LLVM's documentation.

Also rewords -fstack-protector-all for affinity.

Differential Revision: https://reviews.llvm.org/D55428

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@349335 91177308-0d34-0410-b5e6-96231b3b80d8

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