GN documentation updates.

tools/gn/function_process_file_template.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/file_template.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/parse_tree.h"
 
 namespace functions {
 
 const char kProcessFileTemplate[] = "process_file_template";
 const char kProcessFileTemplate_HelpShort[] =
     "process_file_template: Do template expansion over a list of files.";
 const char kProcessFileTemplate_Help[] =
     "process_file_template: Do template expansion over a list of files.\n"
     "\n"
     "  process_file_template(source_list, template)\n"
     "\n"
     "  process_file_template applies a template list to a source file list,\n"
     "  returning the result of applying each template to each source. This is\n"
     "  typically used for computing output file names from input files.\n"
     "\n"
+    "  In most cases, get_target_outputs() will give the same result with\n"
+    "  shorter, more maintainable code. This function should only be used\n"
+    "  when that function can't be used (like there's no target or the target\n"
+    "  is defined in another build file).\n"
+    "\n"
     "Arguments:\n"
     "\n"
     "  The source_list is a list of file names.\n"
     "\n"
     "  The template can be a string or a list. If it is a list, multiple\n"
     "  output strings are generated for each input.\n"
     "\n"
     "  The following template substrings are used in the template arguments\n"
     "  and are replaced with the corresponding part of the input file name:\n"
     "\n"
@@ skipping 32 matching lines @@
   FileTemplate file_template(args[1], err);
   if (err->has_error())
     return Value();
 
   Value ret(function, Value::LIST);
   file_template.Apply(args[0], function, &ret.list_value(), err);
   return ret;
 }
 
 }  // namespace functions