Add source collision test, rust_bindgen doc.

Bug: 159064919
Test: Soong tests pass (and fail if error is modified).
Change-Id: Id4f76e53e11e46ae495f9613a2699c598e7daea4
diff --git a/rust/bindgen.go b/rust/bindgen.go
index 859223a..0f657ef 100644
--- a/rust/bindgen.go
+++ b/rust/bindgen.go
@@ -147,6 +147,9 @@
 		&b.Properties)
 }
 
+// rust_bindgen generates Rust FFI bindings to C libraries using bindgen given a wrapper header as the primary input.
+// Bindgen has a number of flags to control the generated source, and additional flags can be passed to clang to ensure
+// the header and generated source is appropriately handled.
 func RustBindgenFactory() android.Module {
 	module, _ := NewRustBindgen(android.HostAndDeviceSupported)
 	return module.Init()