Windows/x64: Document MSVC flags for C++ exception interoperability.

Suggested by Julien Cugnière.

doc/extensions.html

@@ -353,7 +353,9 @@ the toolchain used to compile LuaJIT:
 on the C stack. The contents of the C++ exception object
 pass through unmodified.</li>
 <li>Lua errors can be caught on the C++ side with <tt>catch(...)</tt>.
-The corresponding Lua error message can be retrieved from the Lua stack.</li>
+The corresponding Lua error message can be retrieved from the Lua stack.<br>
+For MSVC for Windows 64 bit this requires compilation of your C++ code
+with <tt>/EHa</tt>.</li>
 <li>Throwing Lua errors across C++ frames is safe. C++ destructors
 will be called.</li>
 </ul>