Update block supports api readme

docs/reference-guides/block-api/block-supports.md

@@ -329,16 +329,28 @@ supports: {
 - Type: `Object`
 - Default value: null
 - Subproperties:
- - `padding`: type `boolean`, default value `false`
+ - `padding`: type `boolean` or `array`, default value `false`
 
 This value signals that a block supports some of the CSS style properties related to spacing. When it does, the block editor will show UI controls for the user to set their values, if [the theme declares support](/docs/how-to-guides/themes/theme-support.md##cover-block-padding).
 
 ```js
 supports: {
- padding: true, // Enable padding color UI control.
+ spacing: {
+ padding: true, // Enable padding UI control.
+ }
 }
 ```
 
 When the block declares support for a specific spacing property, the attributes definition is extended to include the `style` attribute.
 
 - `style`: attribute of `object` type with no default assigned. This is added when `padding` support is declared. It stores the custom values set by the user.
+
+```js
+supports: {
+ spacing: {
+ padding: [ 'top', 'bottom' ], // Enable padding for arbitrary sides.
+ }
+}
+```
+
+A spacing property may define an array of allowable sides that can be configured. When arbitrary sides are defined only UI controls for those sides are displayed.