Highlight inclusion of extern macro (qmk#19614)

docs/feature_leader_key.md

@@ -41,14 +41,20 @@ As you can see, you have a few functions. You can use `SEQ_ONE_KEY` for single-k
 
 Each of these accepts one or more keycodes as arguments. This is an important point: You can use keycodes from **any layer on your keyboard**. That layer would need to be active for the leader macro to fire, obviously.
 
-## Adding Leader Key Support in the `rules.mk`
+## Adding Leader Key Support
 
-To add support for Leader Key you simply need to add a single line to your keymap's `rules.mk`:
+To enable Leader Key, add the following line to your keymap's `rules.mk`:
 
 ```make
 LEADER_ENABLE = yes
 ```
 
+Place the following macro in your `keymap.c` or user space source file, before any functional code. It handles declaration of external variables that will be referenced by Leader Key codes that follows:
+
+```c
+LEADER_EXTERNS();
+```
+
 ## Per Key Timing on Leader keys
 
 Rather than relying on an incredibly high timeout for long leader key strings or those of us without 200wpm typing skills, we can enable per key timing to ensure that each key pressed provides us with more time to finish our stroke. This is incredibly helpful with leader key emulation of tap dance (read: multiple taps of the same key like C, C, C).