Add forward_variables_from() code example to help text

Change-Id: Id90ced4ceaa5dc62369a97f9792aa61305049c90
Reviewed-on: https://gn-review.googlesource.com/2780
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index d73a94f..c7215df 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -1901,6 +1901,13 @@
 #### **Examples**
 
 ```
+  # forward_variables_from(invoker, ["foo"])
+  # is equivalent to:
+  assert(!defined(foo))
+  if (defined(invoker.foo)) {
+    foo = invoker.foo
+  }
+
   # This is a common action template. It would invoke a script with some given
   # parameters, and wants to use the various types of deps and the visibility
   # from the invoker if it's defined. It also injects an additional dependency

tools/gn/function_forward_variables_from.cc

diff --git a/tools/gn/function_forward_variables_from.cc b/tools/gn/function_forward_variables_from.cc
index de08072..875af91 100644
--- a/tools/gn/function_forward_variables_from.cc
+++ b/tools/gn/function_forward_variables_from.cc
@@ -117,6 +117,13 @@
 
 Examples
 
+  # forward_variables_from(invoker, ["foo"])
+  # is equivalent to:
+  assert(!defined(foo))
+  if (defined(invoker.foo)) {
+    foo = invoker.foo
+  }
+
   # This is a common action template. It would invoke a script with some given
   # parameters, and wants to use the various types of deps and the visibility
   # from the invoker if it's defined. It also injects an additional dependency