[docs][LLVM-style RTTI] Add a mention of multiple inheritance.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228479 91177308-0d34-0410-b5e6-96231b3b80d8

docs/HowToSetUpLLVMStyleRTTI.rst

@@ -377,6 +377,20 @@ contract for ``classof`` is "return ``true`` if the dynamic type of the
 argument is-a ``C``".  As long as your implementation fulfills this
 contract, you can tweak and optimize it as much as you want.
 
+For example, LLVM-style RTTI can work fine in the presence of
+multiple-inheritance by defining an appropriate ``classof``.
+An example of this in practice is
+`Decl <http://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_ vs.
+`DeclContext <http://clang.llvm.org/doxygen/classclang_1_1DeclContext.html>`_
+inside Clang.
+The ``Decl`` hierarchy is done very similarly to the example setup
+demonstrated in this tutorial.
+The key part is how to then incorporate ``DeclContext``: all that is needed
+is in ``bool DeclContext::classof(const Decl *)``, which asks the question
+"Given a ``Decl``, how can I determine if it is-a ``DeclContext``?".
+It answers this with a simple switch over the set of ``Decl`` "kinds", and
+returning true for ones that are known to be ``DeclContext``'s.
+
 .. TODO::
 
    Touch on some of the more advanced features, like ``isa_impl`` and