Help: Add a 3.9 release note about the VS GenerateDebugInformation update

Add a release note corresponding to the environmental regression fixed
by commit v3.9.0-rc6~15^2~1 (VS: Fix GenerateDebugInformation values for
v140 and v141 toolsets, 2017-06-27).

Help/release/3.9.rst

@@ -319,3 +319,8 @@ Other Changes
 
 * On macOS, ``RPATH`` settings such as :prop_tgt:`BUILD_WITH_INSTALL_RPATH`
   no longer affect the ``install_name`` field.  See policy :policy:`CMP0068`.
+
+* The :generator:`Visual Studio 14 2015` generator has been taught about
+  a change to the ``v140`` toolset made by a VS 2015 update.  VS changed
+  the set of values it understands for the ``GenerateDebugInformation``
+  linker setting that produces the ``-DEBUG`` linker flag variants.