docs: better explanation for base configuration (#8779)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

.changeset/thin-flies-notice.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Enriches the explanation of the `base` configuration with examples.

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

@@ -580,7 +580,28 @@ export interface AstroUserConfig {
 	 *
 	 * When using this option, all of your static asset imports and URLs should add the base as a prefix. You can access this value via `import.meta.env.BASE_URL`.
 	 *
-	 * The value of `import.meta.env.BASE_URL` respects your `trailingSlash` config and will include a trailing slash if you explicitly include one or if `trailingSlash: "always"` is set. If `trailingSlash: "never"` is set, `BASE_URL` will not include a trailing slash, even if `base` includes one.
+	 * The value of `import.meta.env.BASE_URL` will be determined by your `trailingSlash` config, no matter what value you have set for `base`.
+	 *
+	 * A trailing slash is always included if `trailingSlash: "always"` is set. If `trailingSlash: "never"` is set, `BASE_URL` will not include a trailing slash, even if `base` includes one.
+	* 
+	* Additionally, Astro will internally manipulate the configured value of `config.base` before making it available to integrations. The value of `config.base` as read by integrations will also be determined by your `trailingSlash` configuration in the same way.
+	 *
+	 	 * In the example below, the values of `import.meta.env.BASE_URL` and `config.base` when processed will both be `/docs`:
+	 * ```js
+	 * {
+	 * 	 base: '/docs/',
+	 * 	 trailingSlash: "never"
+	 * }
+	 * ```
+	 *
+	 * In the example below, the values of `import.meta.env.BASE_URL` and `config.base` when processed will both be `/docs/`:
+	 *
+	 * ```js
+	 * {
+	 * 	 base: '/docs',
+	 * 	 trailingSlash: "always"
+	 * }
+	 * ```
 	 */
 	base?: string;