update icon fieldtype docs

closes #1816

Author: duncanmcclean

content/collections/fieldtypes/icon.md

@@ -9,15 +9,9 @@ screenshot_dark: fieldtypes/screenshots/v6/icon-dark.webp
 options:
   -
     id: nKVDUK7I
-    name: directory
+    name: set
     type: string
-    description: 'Optionally set the path to the directory containing desired icons relative to the project’s root. Default: uses system icons.'
-    required: false
-  -
-    id: 752XSotc
-    name: folder
-    type: string
-    description: 'Optionally set a subdirectory contain a specific set of icons.'
+    description: "Name of a custom icon set. Uses Statamic's icon set by default."
     required: false
   -
     id: u8yCCuXb
@@ -53,3 +47,21 @@ Icon fields return inline string of the selected SVG icon.
 ```html
 <svg viewBox="0 0 24 24"><path fill="currentColor" d="M11.983 0a12.206 12.206 0 0 0-8.51 3.653A11.8 11.8 0 0 0 0 12.207C-.008 18.712 5.26 23.992 11.765 24h.249c6.678-.069 12.04-5.531 11.986-12.209C24.015 5.293 18.76.013 12.262-.003L11.983 0zM10.5 16.542a1.475 1.475 0 0 1 1.421-1.529l.028-.001h.027c.82.002 1.492.651 1.523 1.47a1.475 1.475 0 0 1-1.419 1.529l-.03.001h-.027a1.53 1.53 0 0 1-1.523-1.47zM11 12.5v-6a1 1 0 0 1 2 0v6a1 1 0 0 1-2 0z"></path></svg>
 ```
+
+## Icon Sets
+
+By default, the Icon fieldtype uses Statamic's built-in icons. However, you can register a custom icon set in your `AppServiceProvider` and specify it in the field's config.
+
+```php
+// AppServiceProvider.php
+use Statamic\Facades\Icon; // [tl! ++]
+Icon::register('heroicons', base_path('resources/heroicons')); // [tl! ++]
+```
+
+```yaml
+-
+  handle: favourite_icon
+  field:
+    type: icon
+    set: heroicons # [tl! ++]
+```