(GH-10140) Document env vars in manifests

[michaeltlombardi]

PR Summary

Prior to this change, the documentation noted specific variables you can use in module manifests, but did not note that you can use environment variables, unlike in restricted language mode.

This change:

• Adds a note in the section on module manifest variables to both the about_Module_Manifests and about_Language_Modes articles.
• Resolves Module manifests allow environment variables #10140
• Fixes AB#98137

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.

[learn-build-service-prod]

Learn Build status updates of commit 85df8e2:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Language_Modes.md	✅Succeeded	View (powershell-5.1)
reference/5.1/Microsoft.PowerShell.Core/About/about_Module_Manifests.md	✅Succeeded	View (powershell-5.1)
reference/7.2/Microsoft.PowerShell.Core/About/about_Language_Modes.md	✅Succeeded	View (powershell-7.2)
reference/7.2/Microsoft.PowerShell.Core/About/about_Module_Manifests.md	✅Succeeded	View (powershell-7.2)
reference/7.3/Microsoft.PowerShell.Core/About/about_Language_Modes.md	✅Succeeded	View (powershell-7.3)
reference/7.3/Microsoft.PowerShell.Core/About/about_Module_Manifests.md	✅Succeeded	View (powershell-7.3)
reference/7.4/Microsoft.PowerShell.Core/About/about_Language_Modes.md	✅Succeeded	View (powershell-7.4)
reference/7.4/Microsoft.PowerShell.Core/About/about_Module_Manifests.md	✅Succeeded	View (powershell-7.4)

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