[Docs] Note the order of enumerated custom keycodes (#15706)

Co-authored-by: filterpaper <filterpaper@localhost>

docs/feature_macros.md

@@ -193,6 +193,8 @@ const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
 };
 ```
 
+?> An enumerated list of custom keycodes (`enum custom_keycodes`) must be declared before `keymaps[]` array, `process_record_user()` and any other function that use the list for the compiler to recognise it.
+
 #### Advanced Macros
 
 In addition to the `process_record_user()` function, is the `post_process_record_user()` function. This runs after `process_record` and can be used to do things after a keystroke has been sent.  This is useful if you want to have a key pressed before and released after a normal key, for instance.