test(replay): document package-local scratch fixture

Author: thymikee

packages/replay-port/src/tmp-dir.fixtures.ts

@@ -2,7 +2,8 @@ import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
 
-/** The unit suite redirects TMPDIR to a per-run directory and removes it after each worker. */
+/** The unit suite redirects TMPDIR to a per-run directory removed after all workers finish. */
+// fallow-ignore-next-line code-duplication
 export function mkdtempForTestSync(prefix: string): string {
   return fs.mkdtempSync(path.join(os.tmpdir(), prefix));
 }