Document status of output_required_files command (#12214)

This command is barely functional and exists only for historical
reasons.  State this in the documentation.

Source/cmOutputRequiredFilesCommand.h

@@ -47,8 +47,7 @@ public:
    */
   virtual const char* GetTerseDocumentation() 
     {
-    return 
-      "Output a list of required source files for a specified source file.";
+    return "Deprecated.  Approximate C preprocessor dependency scanning.";
     }
   
   /**
@@ -57,12 +56,22 @@ public:
   virtual const char* GetFullDocumentation()
     {
     return
+      "This command exists only because ancient CMake versions provided it.  "
+      "CMake handles preprocessor dependency scanning automatically using a "
+      "more advanced scanner.\n"
       "  output_required_files(srcfile outputfile)\n"
       "Outputs a list of all the source files that are required by the "
       "specified srcfile. This list is written into outputfile. This is "
       "similar to writing out the dependencies for srcfile except that it "
       "jumps from .h files into .cxx, .c and .cpp files if possible.";
     }
+
+  /** This command is kept for compatibility with older CMake versions. */
+  virtual bool IsDiscouraged()
+    {
+    return true;
+    }
+
   
   cmTypeMacro(cmOutputRequiredFilesCommand, cmCommand);
   void ListDependencies(cmDependInformation const *info,