Add missing reference section to function_toolchain.cc

Update `function_toolchain.cc` to ensure that the new
reference section related to `swiftmodule_switch`, which
was added in [1], does not get lost on the next run
of `tools/update_reference.sh`.

[1] https://gn-review.googlesource.com/c/gn/+/17320

Change-Id: I4a0791d7852acbe9fda73cb980c20c0cb781fbf8
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/17400
Commit-Queue: David Turner <digit@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

src/gn/function_toolchain.cc

diff --git a/src/gn/function_toolchain.cc b/src/gn/function_toolchain.cc
index c42950a..1df3845 100644
--- a/src/gn/function_toolchain.cc
+++ b/src/gn/function_toolchain.cc
@@ -445,6 +445,20 @@
         would be
           "-Wl,-add_ast_path,obj/foo/Foo.swiftmodule"
 
+    rust_swiftmodule_switch [string, optional, link tools only]
+        Valid for: Linker tools except "alink"
+
+        Like swiftmodule_switch, but for targets built/linked with the Rust
+        compiler. The string will be prependend to the path to the
+        .swiftmodule files that are embedded in the linker output.
+
+        If you specified:
+          rust_swiftmodule_swift = "-Clink-arg=-Wl,-add_ast_path,"
+        then the "{{swiftmodules}}" expansion for
+          [ "obj/foo/Foo.swiftmodule" ]
+        would be
+          "-Clink-arg=-Wl,-add_ast_path,obj/foo/Foo.swiftmodule"
+
     outputs  [list of strings with substitutions]
         Valid for: Linker and compiler tools (required)