Document -Wtautological-pointer-compare & -Wtautological-undefined-compare with example

git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_35@215651 91177308-0d34-0410-b5e6-96231b3b80d8
Document -Wabsolute-value with example

git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_35@215652 91177308-0d34-0410-b5e6-96231b3b80d8