Recommendation for installing bash tab completion

### What is the issue?

Recommendation on https://tailscale.com/kb/1080/cli#tab-completion regarding how to save completions is `tailscale completion bash > /etc/bash_completion.d/tailscale`. Recent versions of `bash` prefer `/usr/share/bash-completion/completions/tailscale`, which would be loaded lazily, unlike the current recommended location. I've tested this on Ubuntu 20.04 and a recent update of Arch.

Happy to submit a PR if I only knew where to do it.

### Steps to reproduce

_No response_

### Are there any recent changes that introduced the issue?

No

### OS

Linux

### OS version

Ubuntu 20.04

### Tailscale version

1.72.1

### Other software

_No response_

### Bug report

_No response_