path: add more examples for path.Clean

path: add more examples for path.Clean

Clarify that the function path.Clean only normalises paths and does not
protect against directory-traversal attacks.

Change-Id: I66f1267ac15900ac0cb6011ace0c79aabaebc68b
GitHub-Last-Rev: d669e1edf6221e7c8e0a9aa7bf46493c8ea85ba0
GitHub-Pull-Request: golang/go#74397
Reviewed-on: https://go-review.googlesource.com/c/go/+/684376
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>