[Docs] Recommend the use of RGB light functions for code (#15745)

* Recommend the use of functions instead of keycodes

* Commit suggested revision

Co-authored-by: Nick Brassel <nick@tzarc.org>

Co-authored-by: Nick Brassel <nick@tzarc.org>

docs/feature_rgblight.md

@@ -78,6 +78,7 @@ Changing the **Value** sets the overall brightness.<br>
 
 !> By default, if you have both the RGB Light and the [RGB Matrix](feature_rgb_matrix.md) feature enabled, these keycodes will work for both features, at the same time. You can disable the keycode functionality by defining the `*_DISABLE_KEYCODES` option for the specific feature.
 
+?> `RGB_*` keycodes cannot be used with functions like `tap_code16(RGB_HUI)` as they're not USB HID keycodes. If you wish to replicate similar behaviour in custom code within your firmware (e.g. inside `encoder_update_user()` or `process_record_user()`), the equivalent [RGB functions](#functions-idfunctions) should be used instead.
 
 ## Configuration