GN: Add documentation for gyp command.

tools/gn/command_gyp.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 <fstream>
 
 #include "base/atomicops.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/file_util.h"
@@ skipping 132 matching lines @@
     Err(Location(), "GYP execution failed.", output).PrintToStdout();
     return false;
   }
   return true;
 }
 
 }  // namespace
 
 const char kGyp[] = "gyp";
 const char kGyp_HelpShort[] =
-    "gyp: Run gyp and then GN.";
+    "gyp: Run GYP and then GN.";
 const char kGyp_Help[] =
-    "TODO(brettw) write this.\n";
+    "gyp: Run GYP and then GN.\n"
+    "\n"
+    "  Generate a hybrid GYP/GN build where some targets are generated by\n"
+    "  each of the tools. As long as target names and locations match between\n"
+    "  the two tools, they can depend on each other.\n"
+    "\n"
+    "  When GN is run in this mode, it will not write out any targets\n"
+    "  annotated with \"external = true\". Otherwise, GYP targets with the\n"
+    "  same name and location will be overwritten.\n"
+    "\n"
+    "  References to the GN ninja files will be inserted into the\n"
+    "  GYP-generated build.ninja file.\n"
+    "\n"
+    "Option:\n"
+    "  --no-gyp\n"
+    "      Don't actually run GYP or modify build.ninja. This is used when\n"
+    "      working on the GN build when it is known that no GYP files have\n"
+    "      changed and you want it to run faster.\n";
 
 // Note: partially duplicated from command_gen.cc.
 int RunGyp(const std::vector<std::string>& args) {
   const CommandLine* cmdline = CommandLine::ForCurrentProcess();
   bool no_gyp = cmdline->HasSwitch(kSwitchNoGyp);
 
   // Deliberately leaked to avoid expensive process teardown.
   Setup* setup = new Setup;
   if (!setup->DoSetup())
     return 1;
@@ skipping 53 matching lines @@
         base::IntToString((end_gyp_time - begin_time).InMilliseconds()) +
         "ms)\n";
 
     OutputString(stats);
   }
 
   return 0;
 }
 
 }  // namespace commands