BUG: fix for 8123 documentation issue

Source/cmIncludeExternalMSProjectCommand.h

@@ -66,7 +66,10 @@ public:
       "  include_external_msproject(projectname location\n"
       "                             dep1 dep2 ...)\n"
       "Includes an external Microsoft project in the generated workspace "
-      "file.  Currently does nothing on UNIX.";
+      "file.  Currently does nothing on UNIX. This will create a "
+      "target named INCLUDE_EXTERNAL_MSPROJECT_[projectname].  This can "
+      "be used in the add_dependencies command to make things depend "
+      "on the external project.";
     }
   
   cmTypeMacro(cmIncludeExternalMSProjectCommand, cmCommand);