slices: clarify Insert behavior if i == len(s)

slices: clarify Insert behavior if i == len(s)

Fixes #69104

Change-Id: Ie928239d5ef198ce36759ec4cffb7f6320ff3b82
Reviewed-on: https://go-review.googlesource.com/c/go/+/609175
Reviewed-by: Eli Bendersky <eliben@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>