tools: add a high level explanation for WoA EH data

Add a brief explanation of the data section layout for the unwind data that the
Windows on ARM EH models.  This is simply to provide a rough idea of the layout
of the code involved in the decoding of the unwinding.  Details on the involved
data structures are available in the associated support header.  The bulk of it
is related to printing out the byte-code to help validate generation of WoA EH.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210397 91177308-0d34-0410-b5e6-96231b3b80d8