Add leader_add_user callback (#24266)

docs/features/leader_key.md

@@ -154,6 +154,21 @@ User callback, invoked when the leader sequence ends.
 
 ---
 
+### `bool leader_add_user(uint16_t keycode)` {#api-leader-add-user}
+
+User callback, invoked when a keycode is added to the leader sequence.
+
+#### Arguments {#api-leader-add-user-arguments}
+
+ - `uint16_t keycode`  
+   The keycode to added to the leader sequence.
+
+#### Return Value {#api-leader-add-user-return}
+
+`true` to finish the key sequence, `false` to continue.
+
+---
+
 ### `void leader_start(void)` {#api-leader-start}
 
 Begin the leader sequence, resetting the buffer and timer.