Correct the Vaultik.UI doc comment about --cron (closes #84)

The field comment claimed the cli layer replaces the UI writer with a
discarding writer in --cron mode. It does not: the writer is created
once as ui.New(os.Stdout) and never reassigned, and setupGlobals calls
UI.SetQuiet(true) instead. Quiet mode drops Begin, Complete, Info,
Notice, Detail, Progress, and Banner, but Warningf and Errorf have no
quiet check and always emit, so --cron does not make the writer silent.

The README's --cron bullet had the adjacent understatement ("Silent
unless error"), and now names warnings as well.

Comments and documentation only; no behavior change.

README.md

@@ -168,7 +168,8 @@ needed.
   (System Settings → Privacy & Security → Full Disk Access) to read
   TCC-protected directories; without it the backup aborts with a permission
   error that explains how to fix it
-* `--cron`: Silent unless error (for crontab)
+* `--cron`: Silent on total success; warnings and errors are still printed
+  (for crontab)
 * `--prune`: After backup, drop older snapshots of each backed-up name and
   remove orphaned blobs from remote storage. By default keeps only the latest
   snapshot per name; use `--keep-newer-than` for a rolling window.