[Docs] update feature_rgblight.md: add some figures (#5578)

* Added diagrams to illustrate the RGBLIGHT_LED_MAP macro and the `rgblight_set_clipping_range()` function.

* change fig size

* change fig size

* update figs

docs/feature_rgblight.md

@@ -188,6 +188,7 @@ If you need to change your RGB lighting in code, for example in a macro to chang
 |`rgblight_increase_val_noeeprom()`          |Increase the value for all LEDs. This wraps around at maximum value (not written to EEPROM)                                                                   |
 |`rgblight_decrease_val()`                   |Decrease the value for all LEDs. This wraps around at minimum value                                                                                           |
 |`rgblight_decrease_val_noeeprom()`          |Decrease the value for all LEDs. This wraps around at minimum value (not written to EEPROM)                                                                   |
+|`rgblight_set_clipping_range(pos, num)`     |Set clipping Range                                                                                                                                            |
 
 Additionally, [`rgblight_list.h`](https://github.com/qmk/qmk_firmware/blob/master/quantum/rgblight_list.h) defines several predefined shortcuts for various colors. Feel free to add to this list!
 
@@ -195,15 +196,19 @@ Additionally, [`rgblight_list.h`](https://github.com/qmk/qmk_firmware/blob/maste
 
 If you want to make the logical order of LEDs different from the electrical connection order, you can do this by defining the `RGBLIGHT_LED_MAP` macro in your `config.h`.
 
+Normally, the contents of the LED buffer are output to the LEDs in the same order.
+<img src="https://user-images.githubusercontent.com/2170248/55743718-01866c80-5a6e-11e9-8134-25419928327a.JPG" alt="simple dicrect" width="50%"/>
+
 By defining `RGBLIGHT_LED_MAP` as in the example below, you can specify the LED with addressing in reverse order of the electrical connection order.
 
 ```c
 // config.h
 
-#define RGBLED_NUM 10
+#define RGBLED_NUM 4
-#define RGBLIGHT_LED_MAP { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 }
+#define RGBLIGHT_LED_MAP { 3, 2, 1, 0 }
 
 ```
+<img src="https://user-images.githubusercontent.com/2170248/55743725-08ad7a80-5a6e-11e9-83ed-126a2b0209fc.JPG" alt="simple mapped" width="50%"/>
 
 For keyboards that use the RGB LEDs as a backlight for each key, you can also define it as in the example below.
 
@@ -237,7 +242,28 @@ For keyboards that use the RGB LEDs as a backlight for each key, you can also de
   29, 24, 19, 14,  9,  4 )
 
 ```
+## Clipping Range
 
-## Hardware Modification
+Using the `rgblight_set_clipping_range()` function, you can prepare more buffers than the actual number of LEDs, and output some of the buffers to the LEDs. This is useful if you want the split keyboard to treat left and right LEDs as logically contiguous.
+
+You can set the Clipping Range by executing the following code.
+
+```c
+// some soruce
+  rgblight_set_clipping_range(3, 4);
+```
+<img src="https://user-images.githubusercontent.com/2170248/55743785-2bd82a00-5a6e-11e9-9d4b-1b4ffaf4932b.JPG" alt="clip direct" width="70%"/>
+
+In addition to setting the Clipping Range, you can use `RGBLIGHT_LED_MAP` together.
+
+```c
+// config.h
+#define RGBLED_NUM 8
+#define RGBLIGHT_LED_MAP { 7, 6, 5, 4, 3, 2, 1, 0 }
+
+// some soruce
+  rgblight_set_clipping_range(3, 4);
+```
+<img src="https://user-images.githubusercontent.com/2170248/55743747-119e4c00-5a6e-11e9-91e5-013203ffae8a.JPG" alt="clip mapped" width="70%"/>
 
 If your keyboard lacks onboard underglow LEDs, you may often be able to solder on an RGB LED strip yourself. You will need to find an unused pin to wire to the data pin of your LED strip. Some keyboards may break out unused pins from the MCU to make soldering easier. The other two pins, VCC and GND, must also be connected to the appropriate power pins.