Add notes about SecureString

[sdwheeler]

PR Summary

Add notes about SecureString

• Fixes AB#250852

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 ac19fda:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/security/security-features.md	✅Succeeded	View (>=powershell-5.1)

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

[learn-build-service-prod]

Learn Build status updates of commit d521dd3:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/security/security-features.md	✅Succeeded	View (>=powershell-5.1)

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