Document Fortran_MODULE_DIRECTORY as OUTPUT only (#13034)

CMake does not automatically pass the Fortran module output directory
location to the module search path.  Add a note to the documentation
that the location must also be specified by INCLUDE_DIRECTORIES.

Source/cmTarget.cxx

@@ -982,7 +982,12 @@ void cmTarget::DefineProperties(cmake *cm)
      "When this property is not set the modules will be placed in the "
      "build directory corresponding to the target's source directory.  "
      "If the variable CMAKE_Fortran_MODULE_DIRECTORY is set when a target "
-     "is created its value is used to initialize this property.");
+     "is created its value is used to initialize this property."
+     "\n"
+     "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 "
+     "INCLUDE_DIRECTORIES regardless of this property.");
 
   cm->DefineProperty
     ("GNUtoMS", cmProperty::TARGET,