[Docs] Clarify placement and usage of RGB matrix effect inc files (#15572)

Co-authored-by: Ryan <fauxpark@gmail.com>
Co-authored-by: filterpaper <filterpaper@localhost>

docs/feature_rgb_matrix.md

@@ -554,12 +554,11 @@ In order to change the delay of temperature decrease define
 
 ## Custom RGB Matrix Effects :id=custom-rgb-matrix-effects
 
-By setting `RGB_MATRIX_CUSTOM_USER` (and/or `RGB_MATRIX_CUSTOM_KB`) in `rules.mk`, new effects can be defined directly from userspace, without having to edit any QMK core files.
+By setting `RGB_MATRIX_CUSTOM_USER = yes` in `rules.mk`, new effects can be defined directly from your keymap or userspace, without having to edit any QMK core files.
 
-To declare new effects, create a new `rgb_matrix_user/kb.inc` that looks something like this:
+To declare new effects, create a `rgb_matrix_user.inc` file in the user keymap directory or userspace folder.
 
-`rgb_matrix_user.inc` should go in the root of the keymap directory.
-`rgb_matrix_kb.inc` should go in the root of the keyboard directory.
+?> Hardware maintainers who want to limit custom effects to a specific keyboard can create a `rgb_matrix_kb.inc` file in the root of the keyboard directory, and add `RGB_MATRIX_CUSTOM_KB = yes` to the keyboard level `rules.mk`.
 
 To use custom effects in your code, simply prepend `RGB_MATRIX_CUSTOM_` to the effect name specified in `RGB_MATRIX_EFFECT()`. For example, an effect declared as `RGB_MATRIX_EFFECT(my_cool_effect)` would be referenced with: