pp-trace user documentation - beginnings

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@193841 91177308-0d34-0410-b5e6-96231b3b80d8
pp-trace user documentation - beginnings

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@193842 91177308-0d34-0410-b5e6-96231b3b80d8