Updating help on PowerShell on Mac
At this time, it does not look like the Update-Help Cmdlet is supported for Mac or Linux. For the time being I would suggest going to the technet website for each help file.
You can get there from the Command Line with the -Online parameter:
Get-Help Write-Output -Online or
Get-Help about_Functions -Online These commands will take you directly to technet page you need. These pages will always be up to date.