Mention in docs that custom keys cannot overide default ones

src/style.rs

@@ -145,6 +145,8 @@ impl ProgressStyle {
     }
 
     /// Adds a custom key that references a `&ProgressState` to the template
+    ///
+    /// Note that custom keys cannot override built-in template keys.
     pub fn with_key(mut self, key: &'static str, f: Format) -> ProgressStyle {
         self.format_map.0.insert(key, f);
         self