Document LED physical location index for Planck and Preonic (#12147)

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

keyboards/planck/rev6/rev6.c

@@ -34,6 +34,11 @@ led_config_t g_led_config = { {
   LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL,
   LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL
 } };
+
+// LED physical location index
+//     6 5 4 3
+//        0
+//     7 8 1 2
 #endif
 
 void matrix_init_kb(void) {

keyboards/preonic/rev3/rev3.c

@@ -36,6 +36,11 @@ led_config_t g_led_config = { {
     LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL,
     LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL, LED_FLAG_ALL
 } };
+
+// LED physical location index
+//     6 5 4 3
+//        0
+//     7 8 1 2
 #endif
 
 void matrix_init_kb(void) {