Update config base type description (#4954)

packages/astro/src/@types/astro.ts

@@ -426,6 +426,15 @@ export interface AstroUserConfig {
 	 *   base: '/docs'
 	 * }
 	 * ```
+	 * 
+	 * When using this option, you should mind that all of your imports will be affected. In this example, all of the imports including static resources and codes should add a prefix `/docs/`.
+	 * 
+	 * For example, if you want to use a image in your Astro component, you need to change it from '/someimg.png' into '/docs/someimg.png'.
+	 * 
+	 * ```astro
+	 * <!-- <img src="/someimg.png"> is not correct. -->
+	 * <img src="/docs/someimg.png">
+	 * ```
 	 */
 	base?: string;