ENH: Add comment about RUNTIME_DIRECTORY

Source/cmInstallTargetsCommand.h

@@ -62,10 +62,12 @@ public:
   virtual const char* GetFullDocumentation()
     {
     return
-      "  INSTALL_TARGETS(<dir> target target)\n"
+      "  INSTALL_TARGETS(<dir> [RUNTIME_DIRECTORY dir] target target)\n"
       "Create rules to install the listed targets into the given directory.  "
       "The directory <dir> is relative to the installation prefix, which "
-      "is stored in the variable CMAKE_INSTALL_PREFIX.";
+      "is stored in the variable CMAKE_INSTALL_PREFIX. If RUNTIME_DIRECTORY "
+      "specified, then on systems with special runtime files (Windows DLL), "
+      "the files will be copied to that directory.";
     }
   
   cmTypeMacro(cmInstallTargetsCommand, cmCommand);