Update PSReadline documentation for new color options.

[MartinGC94]

Summary of the new document or enhancement

• List of articles that need to be updated:
  https://github.com/MicrosoftDocs/PowerShell-Docs/blob/staging/reference/7.1/PSReadLine/Set-PSReadLineOption.md
  https://github.com/MicrosoftDocs/PowerShell-Docs/blob/staging/reference/7.1/PSReadLine/Get-PSReadLineOption.md
• New articles that need to be created:
• Link(s) to related code PR(s) in the PowerShell/PowerShell repo:
  Add color options for Attributes, command arguments and loop labels. PowerShell/PSReadLine#1724
• Link(s) to related issue(s) in the PowerShell/PowerShell repo:

Description of what changed

Set-PSReadLineOption.md:
Add the 3 new valid color keys

Get-PSReadLineOption.md:
Update the example output to show the new colors.

[sdwheeler]

Linked PR over a year old. We can reopen this issue if the PR ever gets merged.