[powershell] corrections and feedback

some small suggestions/fixes for the powershell doc:

# corrections

`Write-Host` is used exactly once and never explained. I suggest changing it to `Write-Output` to conform with the rest of the document

The comment for array-deletion seems wrong. It revers to a command called `del`, but actually used is a method called `Remove`.

# Remove arbitrary elements from a array with "del"
$array.Remove($array[2]) # $array is now [1, 2, 3]

# feedback

I was hoping for a little more regarding env-vars. .e.g. how to set them? what datatype is `$env`? when are they reset (when restarting a new terminal, i guess?)? Is it possible to invoke a single command with with a specific env-variable, without setting that variable for the rest of the shell-session?

in one line a `@` is used instead of `$` for a variable, which was a little confusing. The line was `Invoke-DbaQuery @queryParams`. Does `@` have a special meaning here? Is it a typo?