(GH-116) Clarify New-ExternalHelpCab is Windows-only

[michaeltlombardi]

PR Summary

Prior to this change, the documentation didn't state that the PlatyPS cmdlet New-ExternalHelpCab only works on Windows, despite requiring the MakeCab.exe native command, which is only available on Windows.

This change adds a notice to the cmdlet documentation to clarify this limitation.

• Resolves PlatyPS not clear that making cab files is a windows only feature #116
• Fixes AB#41454

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributors guide.
• Style: This PR adheres to the style guide.

[opbld32]

Learn Build status updates of commit d9aab51:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/ps-modules/PlatyPS/New-ExternalHelpCab.md	✅Succeeded	View (ps-modules)

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