Exception handling docs: Clarify how the llvm.eh.* intrinsics are used

The non-SJLJ and SJLJ intrinsics are generated by the frontend and
backend respectively.

Differential Revision: http://llvm-reviews.chandlerc.com/D3010

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205017 91177308-0d34-0410-b5e6-96231b3b80d8