Don't add implicit deps to copy commands in GN.

tools/gn/function_toolchain.cc

Index: tools/gn/function_toolchain.cc
diff --git a/tools/gn/function_toolchain.cc b/tools/gn/function_toolchain.cc
index 377b92720ad26087819d50d3fd574e47ced9bcab..c27163b943d89c0fb89fe986ca058a1f4889df52 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -159,7 +159,7 @@ const char kTool_Help[] =
     "  Used inside a toolchain definition to define a command to run for a\n"
     "  given file type. See also \"gn help toolchain\".\n"
     "\n"
-    "Command types:\n"
+    "Command types\n"
     "\n"
     "  The following values may be passed to the tool() function for the type\n"
     "  of the command:\n"
@@ -167,6 +167,18 @@ const char kTool_Help[] =
     "  \"cc\", \"cxx\", \"objc\", \"objcxx\", \"asm\", \"alink\", \"solink\",\n"
     "  \"link\", \"stamp\", \"copy\"\n"
     "\n"
+    "Tool-specific notes\n"
+    "\n"
+    "  copy\n"
+    "    The copy command should be a native OS command since it does not\n"
+    "    implement toolchain dependencies (which would enable a copy tool to\n"
+    "    be compiled by a previous step).\n"
+    "\n"
+    "    It is legal for the copy to not update the timestamp of the output\n"
+    "    file (as long as it's greater than or equal to the input file). This\n"
+    "    allows the copy command to be implemented as a hard link which can\n"
+    "    be more efficient.\n"
+    "\n"
     "Command flags\n"
     "\n"
     "  These variables may be specified in the { } block after the tool call.\n"