doc: icon in GM_registerMenuCommand

Author: tophf

content/pages/api/gm.mdx

@@ -380,6 +380,9 @@ Returns the command's `caption` since VM2.12.5 or `id` since VM2.15.9.
       Default: `caption` text since VM2.16.2.
       Default in 2.15.9-2.16.1: a randomly generated string.
 
+    * <Field name="icon?" type="string" comment="since VM2.31.1" />
+      URL of the icon.
+
     * <Field name="title?" type="string" />
       A hint shown in the status bar when hovering the command.