gn: Document gen's --check flag.

tools/gn/command_gen.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 "base/atomicops.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "base/timer/elapsed_timer.h"
@@ skipping 256 matching lines @@
   return false;
 }
 
 }  // namespace
 
 const char kGen[] = "gen";
 const char kGen_HelpShort[] = "gen: Generate ninja files.";
 const char kGen_Help[] =
     R"(gn gen: Generate ninja files.
 
-  gn gen [<ide options>] <out_dir>
+  gn gen [--check] [<ide options>] <out_dir>
 
   Generates ninja files from the current tree and puts them in the given output
   directory.
 
   The output directory can be a source-repo-absolute path name such as:
       //out/foo
   Or it can be a directory relative to the current directory such as:
       out/foo
 
+  "gn gen --check" is the same as running "gn check". See "gn help check"
+  for documentation on that mode.
+
   See "gn help switches" for the common command-line switches.
 
 IDE options
 
   GN optionally generates files for IDE. Possibilities for <ide options>
 
   --ide=<ide_name>
       Generate files for an IDE. Currently supported values:
       "eclipse" - Eclipse CDT settings file.
       "vs" - Visual Studio project/solution files.
@@ skipping 49 matching lines @@
   GN DOES NOT generate Eclipse CDT projects. Instead, it generates a settings
   file which can be imported into an Eclipse CDT project. The XML file contains
   a list of include paths and defines. Because GN does not generate a full
   .cproject definition, it is not possible to properly define includes/defines
   for each file individually. Instead, one set of includes/defines is generated
   for the entire project. This works fairly well but may still result in a few
   indexer issues here and there.
 
 Generic JSON Output
 
-  Dumps target information to JSON file and optionally invokes python script on
-  generated file. See comments at the beginning of json_project_writer.cc and
-  desc_builder.cc for overview of JSON file format.
+  Dumps target information to a JSON file and optionally invokes a
+  python script on the generated file. See the comments at the beginning
+  of json_project_writer.cc and desc_builder.cc for an overview of the JSON
+  file format.
 
   --json-file-name=<json_file_name>
       Overrides default file name (project.json) of generated JSON file.
 
   --json-ide-script=<path_to_python_script>
       Executes python script after the JSON file is generated. Path can be
       project absolute (//), system absolute (/) or relative, in which case the
       output directory will be base. Path to generated JSON file will be first
       argument when invoking script.
 
@@ skipping 80 matching lines @@
             setup->scheduler().input_file_manager()->GetInputFileCount()) +
         " files in " +
         base::Int64ToString(elapsed_time.InMilliseconds()) + "ms\n";
     OutputString(stats);
   }
 
   return 0;
 }
 
 }  // namespace commands