Update spacing support docs for margin

docs/how-to-guides/themes/theme-json.md

@@ -192,6 +192,7 @@ The settings section has the following structure and default values:
       },
       "custom": { ... },
       "spacing": {
+        "customMargin": false, /* true to opt-in */
         "customPadding": false, /* true to opt-in, as in add_theme_support('custom-spacing') */
         "units": [ "px", "em", "rem", "vh", "vw" ], /* filter values, as in add_theme_support('custom-units', ... ) */
       },
@@ -374,6 +375,12 @@ Each block declares which style properties it exposes via the [block supports me
 				"text": "value"
 			},
 			"spacing": {
+				"margin": {
+					"top": "value",
+					"right": "value",
+					"bottom": "value",
+					"left": "value"
+				},
 				"padding": {
 					"top": "value",
 					"right": "value",

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

@@ -329,26 +329,29 @@ supports: {
 -   Type: `Object`
 -   Default value: null
 -   Subproperties:
+    -   `margin`: type `boolean` or `array`, 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: {
     spacing: {
+        margin: true,  // Enable margin UI control.
         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.
+-   `style`: attribute of `object` type with no default assigned. This is added when `margin` or `padding` support is declared. It stores the custom values set by the user.
 
 ```js
 supports: {
     spacing: {
-        padding: [ 'top', 'bottom' ], // Enable padding for arbitrary sides.
+        margin: [ 'top', 'bottom' ], // Enable margin for arbitrary sides.
+        padding: true,               // Enable padding for all sides.
     }
 }
 ```