PR32346 suggests that UBSan's docs about the -fsanitize,
-fno-sanitize-recover, and -fsanitize-trap options are not explicit
enough. Try to improve the wording.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298310 91177308-0d34-0410-b5e6-96231b3b80d8