docs(internal): add module docs (#4757)

internal/mod.ts

@@ -5,6 +5,35 @@
  *
  * Note: this module is for internal use only and should not be used directly.
  *
+ * ```ts
+ * import { diff, diffstr, buildMessage } from "@std/internal";
+ * import { assertEquals } from "@std/assert/assert-equals";
+ *
+ * const a = [1, 2, 3];
+ * const b = [1, 2, 4];
+ *
+ * assertEquals(diff(a, b), [
+ *   { type: "common", value: 1 },
+ *   { type: "common", value: 2 },
+ *   { type: "removed", value: 3 },
+ *   { type: "added", value: 4 },
+ * ]);
+ *
+ * const diffResult = diffstr("Hello, world!", "Hello, world");
+ *
+ * console.log(buildMessage(diffResult));
+ * // [
+ * //   "",
+ * //   "",
+ * //   "    [Diff] Actual / Expected",
+ * //   "",
+ * //   "",
+ * //   "-   Hello, world!",
+ * //   "+   Hello, world",
+ * //   "",
+ * // ]
+ * ```
+ *
  * @module
  */
 export * from "./build_message.ts";