docs(alert): document custom properties (#16251)

references #14850

core/src/components/alert/alert.scss

@@ -4,6 +4,11 @@
 // --------------------------------------------------
 
 :host {
+  /**
+   * @prop --background: Background of the alert
+   * @prop --min-width: Minimum width of the alert
+   * @prop --max-height: Maximum height of the alert
+   */
   --min-width: #{$alert-min-width};
   --max-height: #{$alert-max-height};
 

core/src/components/alert/readme.md

@@ -105,6 +105,15 @@ Type: `Promise<void>`
 
 
 
+## CSS Custom Properties
+
+| Name           | Description                 |
+| -------------- | --------------------------- |
+| `--background` | Background of the alert     |
+| `--max-height` | Maximum height of the alert |
+| `--min-width`  | Minimum width of the alert  |
+
+
 ----------------------------------------------
 
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