Update role assignment cmdlet

[dagoroz]

Description

Added cmdlet "Set-AzRoleAssignment" and small suggestions that arose from the design review

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
• x] 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

Link to review

https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/689

[adxsdkps]

Can one of the admins verify this patch?

[jiasli]

Could you update the New-AzRoleAssignment.md and also add Set-AzRoleAssignment.md to provide more examples on the updated commands?

[isra-fel]

Hi @dagoroz , please CIL.
BTW the help file Set-AzRoleAssignment.md is missing.

Thanks

[isra-fel]

Hi @dagoroz , only thing left is to regenerate help docs: https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/help-generation.md#updating-help-after-making-cmdlet-changes

Some of the changes were not reflected in docs, for example there should be description of -WhatIf parameter in Set-AzRoleAssignment.md