Document syncing layer change callback across halves

docs/feature_layers.md

@@ -179,6 +179,30 @@ Use the `IS_LAYER_ON_STATE(state, layer)` and `IS_LAYER_OFF_STATE(state, layer)`
 
 Outside of `layer_state_set_*` functions, you can use the `IS_LAYER_ON(layer)` and `IS_LAYER_OFF(layer)` macros to check global layer state.
 
+### Running on slave half as well as master (for split keyboards)
+
+By default `layer_state_set_*` functions only run on master. A workaround is to manually update it in housekeeping.
+
+In `keymap.c`:
+
+```c
+static uint32_t last_layer_state = 0;
+
+void update_layer_state_set(void){
+    if (!is_keyboard_master()) {
+        if (last_layer_state != layer_state) {
+            last_layer_state = layer_state;
+            layer_state_set_user(layer_state);
+        }
+    }
+}
+
+void housekeeping_task_user(void) {
+    update_layer_state_set();
+}
+```
+
+
 ### `layer_state_set_*` Function Documentation
 
 * Keyboard/Revision: `layer_state_t layer_state_set_kb(layer_state_t state)`

docs/feature_split_keyboard.md

@@ -313,6 +313,7 @@ QMK's split transport allows for arbitrary data transactions at both the keyboar
 To leverage this, a keyboard or user/keymap can define a comma-separated list of _transaction IDs_:
 
 ```c
+// *_SYNC_* names can be called whatever you want, so long as they are passed to transaction_register_rpc with the function they apply to
 // for keyboard-level data sync:
 #define SPLIT_TRANSACTION_IDS_KB KEYBOARD_SYNC_A, KEYBOARD_SYNC_B
 // or, for user:
@@ -381,6 +382,27 @@ By default, the inbound and outbound data is limited to a maximum of 32 bytes ea
 #define RPC_S2M_BUFFER_SIZE 48
 ```
 
+If you only need to transfer a small integer one way, or just run a function on the slave, this can be simplified to:
+
+```c
+#include "transactions.h"
+
+void func_to_call(uint8_t data, const void* unused, uint8_t unused2, void* unused3) {
+    do_something_with(data);
+}
+void keyboard_post_init_user(void) {
+    transaction_register_rpc(USER_SYNC_A, func_to_call);
+}
+
+void housekeeping_task_user(void) {
+    if (is_keyboard_master()){
+        // 24 is arbitrary int8
+        transaction_rpc_exec(USER_SYNC_A, 24, NULL, 0, NULL);
+    }
+}
+
+```
+
 ###  Hardware Configuration Options
 
 There are some settings that you may need to configure, based on how the hardware is set up.