Add documentation for GN's GYP generation.

tools/gn/variables.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/variables.h"
 
 namespace variables {
 
 // Built-in variables ----------------------------------------------------------
 
@@ skipping 474 matching lines @@
     "      } else {\n"
     "        deps = \"//third_party/fallback_frob\"\n"
     "      }\n"
     "      forward_dependent_configs_from = deps\n"
     "    }\n";
 
 const char kGypFile[] = "gyp_file";
 const char kGypFile_HelpShort[] =
     "gyp_file: [file name] Name of GYP file to write to in GYP mode.";
 const char kGypFile_Help[] =
-    "gyp_file: Name of GYP file to write to in GYP mode.\n";
+    "gyp_file: Name of GYP file to write to in GYP mode.\n"
+    "\n"
+    "  See \"gn help gyp\" for an overview of how this works.\n"
+    "\n"
+    "  Tip: If all targets in a given BUILD.gn file should go in the same\n"
+    "  GYP file, just put gyp_file = \"foo\" at the top of the file and\n"
+    "  the variable will be in scope for all targets.\n";
 
 const char kHardDep[] = "hard_dep";
 const char kHardDep_HelpShort[] =
     "hard_dep: [boolean] Indicates a target should be built before dependees.";
 const char kHardDep_Help[] =
     "hard_dep: Indicates a target should be built before dependees.\n"
     "\n"
     "  Ninja's default is to assume that targets can be compiled\n"
     "  independently. This breaks down for generated files that are included\n"
     "  in other targets because Ninja doesn't know to run the generator\n"
@@ skipping 270 matching lines @@
     INSERT_VARIABLE(Script)
     INSERT_VARIABLE(SourcePrereqs)
     INSERT_VARIABLE(Sources)
   }
   return info_map;
 }
 
 #undef INSERT_VARIABLE
 
 }  // namespace variables