Clarify docs on ShutdownScript::new_witness_program somewhat

This is especially important for C or other language bindings
clients as the `version` field may be exported as a `u8`.

Author: TheBlueMatt

lightning/src/ln/script.rs

@@ -86,7 +86,10 @@ impl ShutdownScript {
 		Self(ShutdownScriptImpl::Bolt2(Script::new_v0_wsh(script_hash)))
 	}
 
-	/// Generates a P2WSH script pubkey from the given segwit version and program.
+	/// Generates a witness script pubkey from the given segwit version and program.
+	///
+	/// Note for version-zero witness scripts you must use [`ShutdownScript::new_p2wpkh`] or
+	/// [`ShutdownScript::new_p2wsh`] instead.
 	///
 	/// # Errors
 	///