[Eng]: should not generate documents and test stubs for internal functions

### Description

There are some internal (helper) functions in Az.Functions module such as `Get-AzSystemAssignedIdentity` or `Test-Az`. Their functionality is not exposed to the end user, which is as excepted, but their documents and test stubs still get generated, costing extra effort to remove them before releasing (for example [this commit](https://github.com/Azure/azure-powershell/pull/18209/commits/ece524ce259e055e937a2e88ef510e58da9deaf5)).

I'm talking about files in these directories (main branch):

- examples
- help
- tests