Updating docs for MSan to describe poison-in-dtor.
Updating docs for MSan to describe poison-in-dtor.

Summary:
Describe the compile and runtime flags to enable MemorySanitizer
detection of use-after-destroy.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12914

Revise doc description of use-after-dtor.

Change wording to specify memory no longer readable.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@247871 91177308-0d34-0410-b5e6-96231b3b80d8

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