Add new examples

[timwarner-msft]

Added examples to the New-AzPolicyAssignment and New-AzPolicyDefinition help files to show customers how to get the reader-friendly DisplayName property from those objects.

Description

Checklist

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD select appropriate branch. Cmdlets from Autorest.PowerShell should go to generation branch.
• SHOULD make the title of PR clear and informative, and in the present imperative tense.
• SHOULD update ChangeLog.md file(s) appropriately
  • 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 in the past tense. Add changelog in description section if PR goes into generation branch.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD have approved design review for the changes in this repository (Microsoft internal only) with following situations
  • Create new module from scratch
  • Create new resource types which are not easy to conform to Azure PowerShell Design Guidelines
  • Create new resource type which name doesn't use module name as prefix
  • Have design question before implementation
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT introduce breaking changes in Az minor release except preview version.
• SHOULD NOT adjust version of module manually in pull request

[timwarner-msft]

cc: @shanhix1, @mikefrobbins

[mikefrobbins]

Use full parameter names instead of positional parameters. Couple of minor style issues, but otherwise looks good.

[msJinLei]

/azp run azure-powershell - security-tools

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[msJinLei]

@mikefrobbins could you help to see whether all of your comments are addressed? thanks

[mikefrobbins]

@msJinLei All of my comments have been addressed and this PR is ready to be merged. #sign-off

[mikefrobbins]

LGTM