action group parameters allow for multiple values

some of these action group related parameters allow for multiple values (i think the delete) but i would really like if multiple values were an option on all action group related parameters. in my case the create.

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: d97968df-72c9-e50c-d5b1-54562c1bdef7
* Version Independent ID: 0cde4f15-6464-3106-b787-3ee37c7de0ee
* Content: [az monitor metrics alert](https://docs.microsoft.com/en-us/cli/azure/monitor/metrics/alert?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/monitor/metrics/alert.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/monitor/metrics/alert.yml)
* Service: **monitoring-and-diagnostics**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**