Fixes #10696 - Add info about using environment variable

[sdwheeler]

PR Summary

Add info about using environment variable

• Fixes AB#187724
• Fixes Make it clear that the POWERSHELL_UPDATECHECK environment variable must be set *before* PowerShell is launched #10696

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 9df9793:

✅ Validation status: passed

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

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
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[learn-build-service-prod]

Learn Build status updates of commit c0c900f:

✅ Validation status: passed

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

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[mklement0]

🙏