Update GN documentation.

tools/gn/command_help.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 <algorithm>
 #include <iostream>
 
 #include "base/command_line.h"
 #include "tools/gn/args.h"
 #include "tools/gn/commands.h"
@@ skipping 135 matching lines @@
   return true;
 }
 
 }  // namespace
 
 const char kHelp[] = "help";
 const char kHelp_HelpShort[] =
     "help: Does what you think.";
 const char kHelp_Help[] =
     "gn help <anything>\n"
+    "\n"
     "  Yo dawg, I heard you like help on your help so I put help on the help\n"
-    "  in the help.\n";
+    "  in the help.\n"
+    "\n"
+    "  You can also use \"all\" as the parameter to get all help at once.\n"
+    "\n"
+    "Switches\n"
+    "\n"
+    "  --markdown\n"
+    "      Format output in markdown syntax.\n"
+    "\n"
+    "Example\n"
+    "\n"
+    "  gn help --markdown all\n"
+    "      Dump all help to stdout in markdown format.\n";
 
 int RunHelp(const std::vector<std::string>& args) {
   std::string what;
   if (args.size() == 0) {
     // If no argument is specified, check for switches to allow things like
     // "gn help --args" for help on the args switch.
     const base::CommandLine::SwitchMap& switches =
         base::CommandLine::ForCurrentProcess()->GetSwitches();
     if (switches.empty()) {
       // Still nothing, show help overview.
@@ skipping 86 matching lines @@
     OutputString("Run `gn help` for a list of available topics.\n",
                  DECORATION_NONE);
   } else {
     OutputString("Did you mean `gn help " + suggestion.as_string() + "`?\n",
                  DECORATION_NONE);
   }
   return 1;
 }
 
 }  // namespace commands