Deprecate the lightning::util::string re-export from types

Like we've done for `features` and `payment`, here we mark the
`lightning::util::string` re-export as deprecated.

lightning/src/util/mod.rs

@@ -56,5 +56,9 @@ pub mod test_channel_signer;
 
 pub mod string {
 	//! Utilities to wrap untrusted strings and handle them (more) safely
+	//!
+	//! These re-exports are deprecated in favor of [`lightning::types::string`].
+	//!
+	//! [`lightning::types::string`]: crate::types::string
 	pub use lightning_types::string::{PrintableString, UntrustedString};
 }