Add warning message for use of positional

crates/nu-cli/src/completions/custom_completions.rs

@@ -105,6 +105,7 @@ impl<T: Completer> Completer for CustomCompletion<T> {
                         if let Some(positional) =
                             options.get("positional").and_then(|val| val.as_bool().ok())
                         {
+                            log::warn!("Use of the positional option is deprecated. Use the substring match algorithm instead.");
                             completion_options.positional = positional;
                         }
                         if let Some(algorithm) = options