Get-Help about_* bugs (conceptual help topics) #3782

Note: This is an attempt to consolidate the related existing issues:

• Get-Help does not list all 'About*' topics  #2653
• Get-Help About_Automatic_Variables does not dislay the full help content #3503
• Get-Help about_Classes does not return help #3728
• Get-Help does not return help about the 'PowerShell Help System' when being called parameterless in PS6-Beta1 on Windows #3781

The behavior as of beta1, verified on Windows 10, macOS 10.12, Ubuntu 16.04:

• Get-Help about_* (using wildcards)

  • Windows: OK
  • Unix: Broken
    • No success output, only error get-help : Unable to find type [wmiclass].
    • Problem starts with as few chars. as Get-Help ab*, but, curiously, doesn't break with Get-Help a*, in which case only matching non-conceptual topics are output.

• Get-Help about_Variable (any individual topic, specified in full, without wildcards)

  • Windows: Broken
    • Shows a list of 2 duplicates rather than the text of the help topic itself.
  • Unix: OK

• Tab completion with about_* topics:

  • Windows: OK
  • Unix: Broken
    • Nothing happens when you press tab after typing Get-Help about_, for instance.