docs: document the properties of the Fresh config

Author: csvn

src/config.ts

@@ -1,20 +1,37 @@
 import * as path from "@std/path";
 
 export interface FreshConfig {
+  /**
+   * The root directory of the Fresh project.
+   *
+   * Other paths, such as `build.outDir`, `staticDir`, and `fsRoutes()`
+   * are resolved relative to this directory.
+   * @default Deno.cwd()
+   */
   root?: string;
   build?: {
     /**
      * The directory to write generated files to when `dev.ts build` is run.
+     *
      * This can be an absolute path, a file URL or a relative path.
+     * Relative paths are resolved against the `root` option.
+     * @default "_fresh"
      */
     outDir?: string;
   };
   /**
    * Serve fresh from a base path instead of from the root.
    *   "/foo/bar" -> http://localhost:8000/foo/bar
-   * @default {undefined}
+   * @default undefined
    */
   basePath?: string;
+  /**
+   * The directory to serve static files from.
+   *
+   * This can be an absolute path, a file URL or a relative path.
+   * Relative paths are resolved against the `root` option.
+   * @default "static"
+   */
   staticDir?: string;
 }