--required-reviewer-ids for az repos policy required-reviewer create command requires UPN not e-mail address

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)
Azure DevOps

### Description of issue (in as much detail as possible)
When specifying required reviewers for the `az repos policy required-reviewer create` using the `--required-reviewer-ids` parameter the documentation specifies these should be email addresses when it should actually be UPNs.

The below set of code does not reliably work when the users UPN and Mail values are not the same.

```
$Readers = $((az ad group member list --group "ado-$ResourceName-reader" | ConvertFrom-Json).mail)

$Contributors = $((az ad group member list --group "ado-$ResourceName-contributor" | ConvertFrom-Json).mail)

$Admins = $((az ad group member list --group "ado-$ResourceName-repo-adm" | ConvertFrom-Json).mail)

$Reviewers = @($Readers) + @($Contributors) + @($Admins) | Sort-Object | Get-Unique

$AppliedPolicy = az repos policy required-reviewer create `
--blocking $false `
--branch "master" `
--enabled $true `
--message "Automatically added as reviewer for by policy for $($Repository.name) repo." `
--repository-id "$($Repository.id)" `
--branch-match-type "exact" `
--org $DevOpsOrganization `
--project $DevOpsProject `
--required-reviewer-ids $($Reviewers -join ";") | ConvertFrom-Json
```

Using the below set of code does work reliably for all users:

```
$Readers = $((az ad group member list --group "ado-$ResourceName-reader" | ConvertFrom-Json).userprincipalname)

$Contributors = $((az ad group member list --group "ado-$ResourceName-contributor" | ConvertFrom-Json).userprincipalname)

$Admins = $((az ad group member list --group "ado-$ResourceName-repo-adm" | ConvertFrom-Json).userprincipalname)

$Reviewers = @($Readers) + @($Contributors) + @($Admins) | Sort-Object | Get-Unique

$AppliedPolicy = az repos policy required-reviewer create `
--blocking $false `
--branch "master" `
--enabled $true `
--message "Automatically added as reviewer for by policy for $($Repository.name) repo." `
--repository-id "$($Repository.id)" `
--branch-match-type "exact" `
--org $DevOpsOrganization `
--project $DevOpsProject `
--required-reviewer-ids $($Reviewers -join ";") | ConvertFrom-Json
```
Can the documentation be updated to reflect this?

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