[WinEH] Add documentation motivating the new EH instructions

This adds documentation on how to use the new EH instructions added in
r243766.

Reviewers: majnemer, reames

Differential Revision: http://reviews.llvm.org/D11565

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244267 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed