Improve PowerShell completion setup documentation

[l0calh05t]

Fixes #3128

[rbtcollins]

Sorry for not reviewing this early. You have a lint failure of some sort - either formatting or clippy - if you could fix this we can merge it I think. Thanks!

[l0calh05t]

Sorry for not reviewing this early. You have a lint failure of some sort - either formatting or clippy - if you could fix this we can merge it I think. Thanks!

If I remember correctly, the error was unrelated to my commit. However, due to the age, the logs had been deleted. I merged the current master to trigger a rerun, now the general checks succeed, but the Linux build failed - again with an error unrelated to my commit:

Run arduino/setup-protoc@v1
Error: API rate limit exceeded for 104.45.202.145. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)

[rbtcollins]

@l0calh05t our UI tests are failing though. These are new since your PR I think.

---- suite::cli_ui::rustup_ui_doc_text_tests stdout ----
thread 'suite::cli_ui::rustup_ui_doc_text_tests' panicked at '1 of 44 tests failed', /cargo/registry/src/github.com-1ecc6299db9ec823/trycmd-0.14.16/src/runner.rs:104:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    suite::cli_ui::rustup_ui_doc_text_tests

[hi-rustin]

@l0calh05t Friendly ping~

Do you have time to update the tests then we can move forward with this PR? Thanks!

[l0calh05t]

I'll try to get around to it this weekend/early next week