SOURCE, COMMAND, TARGET are required now

Source/cmAddCustomCommandCommand.h

@@ -67,7 +67,7 @@ public:
   virtual const char* GetFullDocumentation()
     {
     return
-      "ADD_CUSTOM_COMMAND([SOURCE source] [COMMAND command] [TARGET target] "
+      "ADD_CUSTOM_COMMAND(SOURCE source COMMAND command TARGET target "
       "[ARGS [args...]] [DEPENDS [depends...]] [OUTPUTS [outputs...]])\n"
       "Add a custom command.";
     }