Add forward_variables_from() and target() to GN

tools/gn/function_template.cc

Index: tools/gn/function_template.cc
diff --git a/tools/gn/function_template.cc b/tools/gn/function_template.cc
index 7af688e75b2344c3b1039d231e13c9f2249169c1..1f1da82fb269f2329f8c7699f39fc554fb62631f 100644
--- a/tools/gn/function_template.cc
+++ b/tools/gn/function_template.cc
@@ -28,7 +28,7 @@ const char kTemplate_Help[] =
     "  other files will import (see \"gn help import\") so your template\n"
     "  rule can be shared across build files.\n"
     "\n"
-    "More details:\n"
+    "Variables and templates:\n"
     "\n"
     "  When you call template() it creates a closure around all variables\n"
     "  currently in scope with the code in the template block. When the\n"
@@ -44,6 +44,19 @@ const char kTemplate_Help[] =
     "  that code specifies the file names. This means all files internal\n"
     "  to the template should use absolute names.\n"
     "\n"
+    "  A template will typically forward some or all variables from the\n"
+    "  invoking scope to a target that it defines. Often, such variables\n"
+    "  might be optional. Use the pattern:\n"
+    "\n"
+    "    if (defined(invoker.deps)) {\n"
+    "      deps = invoker.deps\n"
+    "    }\n"
+    "\n"
+    "  The function forward_variables_from() provides a shortcut to forward\n"
+    "  one or more or possibly all variables in this manner:\n"
+    "\n"
+    "    forward_variables_from(invoker, [\"deps\", \"public_deps\"])\n"
+    "\n"
     "Target naming:\n"
     "\n"
     "  Your template should almost always define a built-in target with the\n"