Add more documentation for GN header checking

tools/gn/commands.h

 // 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.
 
 #ifndef TOOLS_GN_COMMANDS_H_
 #define TOOLS_GN_COMMANDS_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 158 matching lines @@
 // Applies any testonly and type filters specified on the command line,
 // and prints the targets as specified by the --as command line flag.
 //
 // If indent is true, the results will be indented two spaces.
 //
 // The vector will be modified so that only the printed targets will remain.
 void FilterAndPrintTargets(bool indent, std::vector<const Target*>* targets);
 void FilterAndPrintTargetSet(bool indent,
                              const std::set<const Target*>& targets);
 
+// Extra help from command_check.cc
+extern const char kNoGnCheck_Help[];
+
 }  // namespace commands
 
 #endif  // TOOLS_GN_COMMANDS_H_