add documentation mentions of cranelift-object where appropriate

This change makes it slightly more obvious that `cranelift-object` can
be used in lieu of `cranelift-faerie`.

cranelift/module/src/backend.rs

@@ -17,13 +17,16 @@ use std::string::String;
 
 /// A `Backend` implements the functionality needed to support a `Module`.
 ///
-/// Two notable implementations of this trait are:
+/// Three notable implementations of this trait are:
 ///  - `SimpleJITBackend`, defined in [cranelift-simplejit], which JITs
 ///    the contents of a `Module` to memory which can be directly executed.
+///  - `ObjectBackend`, defined in [cranelift-object], which writes the
+///    contents of a `Module` out as a native object file.
 ///  - `FaerieBackend`, defined in [cranelift-faerie], which writes the
 ///    contents of a `Module` out as a native object file.
 ///
 /// [cranelift-simplejit]: https://docs.rs/cranelift-simplejit/
+/// [cranelift-object]: https://docs.rs/cranelift-object/
 /// [cranelift-faerie]: https://docs.rs/cranelift-faerie/
 pub trait Backend
 where