Help: Expand Fortran_MODULE_DIRECTORY documentation.

Document how Visual Studio generators append per-configuration directory
to path where Fortran modules are created.

Help/prop_tgt/Fortran_MODULE_DIRECTORY.rst

@@ -11,6 +11,14 @@ corresponding to the target's source directory.  If the variable
 :variable:`CMAKE_Fortran_MODULE_DIRECTORY` is set when a target is created its
 value is used to initialize this property.
 
+When using one of the :ref:`Visual Studio Generators` with the Intel Fortran
+plugin installed in Visual Studio, a subdirectory named after the
+configuration will be appended to the path where modules are created.
+For example, if ``Fortran_MODULE_DIRECTORY`` is set to ``C:/some/path``,
+modules will end up in ``C:/some/path/Debug`` (or
+``C:/some/path/Release`` etc.) when an Intel Fortran ``.vfproj`` file is
+generated, and in ``C:/some/path`` when any other generator is used.
+
 Note that some compilers will automatically search the module output
 directory for modules USEd during compilation but others will not.  If
 your sources USE modules their location must be specified by