add documentation of transparency

Author: matt-land

shared-bindings/displayio/Palette.c

@@ -68,14 +68,16 @@ STATIC mp_obj_t displayio_palette_make_new(const mp_obj_type_t *type, size_t n_a
 }
 //|   .. method:: __setitem__(index, value)
 //|
-//|     Sets the pixel color at the given index. The index should be an integer in the range 0 to color_count-1
+//|     Sets the pixel color at the given index. The index should be an integer in the range 0 to color_count-1.
 //|
-//|     The color can be from 0x000000 to 0xFFFFFF, and can be an int or bytes (3 bytes (RGB) or 4 bytes (RGB + pad byte))
+//|     The value argument represents a color, and can be from 0x000000 to 0xFFFFFF (to represent an RGB value), 
+//|     or None to represent transparency. Value can be an int or bytes (3 bytes (RGB) or 4 bytes (RGB + pad byte)).
 //|
 //|     This allows you to::
 //|
 //|       palette[0] = 0xFFFFFF
 //|       palette[1] = 0xFF0000
+//|       palette[2] = None  # transparency
 //|
 STATIC mp_obj_t palette_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value) {
     if (value == MP_OBJ_NULL) {