[wwwdocs] Add a case to porting_to + a question wrt validity of another one

[Marek Polacek]

This patch adds a description of something I noticed while doing the 
Fedora mass rebuild.  Do we want to say more about the invalidity of
the incomplete type case?

Furthermore I noticed that the testcase below doesn't compile anymore since
r240874; Nathan, is that desirable?

class C {
public:
  virtual ~C() noexcept(false);
};
class A : virtual C {
  virtual C m_fn1() = 0;
};
class B : A {};

s.cc:8:7: error: looser throw specifier for ‘virtual B::~B() noexcept (false)’
 class B : A {};
       ^
s.cc:5:7: error:   overriding ‘virtual A::~A() noexcept’
 class A : virtual C {
       ^

If it's desirable that g++ rejects this, we should say something about it in
porting_to.  Any ideas what that might be?

Thanks,

Index: gcc-7/porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-7/porting_to.html,v
retrieving revision 1.3
diff -u -r1.3 porting_to.html
--- gcc-7/porting_to.html       3 Feb 2017 07:55:27 -0000       1.3
+++ gcc-7/porting_to.html       6 Feb 2017 17:08:44 -0000
@@ -33,6 +33,23 @@
 
 <h2 id="cxx">C++ language issues</h2>
 
+<h3 id="incomplete-type">Invalid use of incomplete type</h3>
+
+<p>
+GCC 7 no longer accepts ill-formed code involving use of an incomplete type:
+<pre><code>
+namespace N {
+class C;
+class A {
+  C fn1();
+};
+template <typename> class B : A {
+  void fn2() { fn1().x; }
+};
+}
+</code></pre>
+</p>
+
 <h3 id="conversion-op-mangling">Mangling change for conversion operators</h3>
 
 <p>

        Marek