VS: Document VS_GLOBAL_KEYWORD and VS_KEYWORD relationship (#14493)

These two target properties serve the same purpose for different versions
of Visual Studio.  Document the versions covered by each property.

Reported-by: [email protected]

Help/prop_tgt/VS_GLOBAL_KEYWORD.rst

@@ -1,9 +1,12 @@
 VS_GLOBAL_KEYWORD
 -----------------
 
-Visual Studio project keyword.
+Visual Studio project keyword for VS 10 (2010) and newer.
 
 Sets the "keyword" attribute for a generated Visual Studio project.
 Defaults to "Win32Proj".  You may wish to override this value with
 "ManagedCProj", for example, in a Visual Studio managed C++ unit test
 project.
+
+Use the :prop_tgt:`VS_KEYWORD` target property to set the
+keyword for Visual Studio 9 (2008) and older.

Help/prop_tgt/VS_KEYWORD.rst

@@ -1,7 +1,10 @@
 VS_KEYWORD
 ----------
 
-Visual Studio project keyword.
+Visual Studio project keyword for VS 9 (2008) and older.
 
 Can be set to change the visual studio keyword, for example Qt
 integration works better if this is set to Qt4VSv1.0.
+
+Use the :prop_tgt:`VS_GLOBAL_KEYWORD` target property to set the
+keyword for Visual Studio 10 (2010) and newer.