Fixes #11017 - Clarify syntax of substitution expressions

[sdwheeler]

PR Summary

Clarify syntax of substitution expressions

• Fixes AB#245895
• Fixes Description of the syntax of -replace's substitution operand is potentially misleading #11017

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 53288b1:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Regular_Expressions.md	✅Succeeded	View (powershell-5.1)
reference/7.2/Microsoft.PowerShell.Core/About/about_Regular_Expressions.md	✅Succeeded	View (powershell-7.2)
reference/7.3/Microsoft.PowerShell.Core/About/about_Regular_Expressions.md	✅Succeeded	View (powershell-7.3)
reference/7.4/Microsoft.PowerShell.Core/About/about_Regular_Expressions.md	✅Succeeded	View (powershell-7.4)
reference/7.5/Microsoft.PowerShell.Core/About/about_Regular_Expressions.md	✅Succeeded	View (powershell-7.5)

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[mklement0]

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