New-AzVm and New-AzVmConfig licenseType description edits

[Sandido]

Work for https://msazure.visualstudio.com/One/_workitems/edit/8323663.
The AzVM documentation in PS had similar incorrect text stating the LicenseType cannot be updated, when in fact it can be.
The VMSS documentation had no mention of LicenseType and so is not included here.
I improved the New-AzVmConfig LicenseType description while I was in the area.

Description

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here

[grizzlytheodore]

lgtm

[isra-fel]

LGTM too. @Sandido let me know when it's ready to merge please

[isra-fel]

/azp run azure-powershell - windows-powershell

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Sandido]

Ready to merge.

[isra-fel]

/azp run azure-powershell - powershell-core

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).