GN: Add documentation for gyp command.

tools/gn/command_gyp.cc

Index: tools/gn/command_gyp.cc
diff --git a/tools/gn/command_gyp.cc b/tools/gn/command_gyp.cc
index 11fd26934ac8f9020377f58b92ca28cbd9e3c8a9..0881291d2439745752a037cddb4bd93741a30463 100644
--- a/tools/gn/command_gyp.cc
+++ b/tools/gn/command_gyp.cc
@@ -150,9 +150,26 @@ bool RunGyp(const BuildSettings* build_settings) {
 
 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) {