Update SafeHandle guidance to recommend adding a public constructor

[jkoritzinsky]

Summary

We've updated our guidance for SafeHandle-derived types. Update the docs to match the new guidance.

Contributes to dotnet/runtime#46830

[dotnet-issue-labeler]

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[opbld34]

Docs Build status updates of commit 9760b24:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
	❌Error		Details

• Line 0, Column 0: [Error-git-clone-failed] Failure to clone the repository `https://github.com/Microsoft/templates.docs.msft#master`. This could be caused by an incorrect repository URL, please verify the URL on the Docs Portal (https://ops.microsoft.com). This could also be caused by not having the proper permission the repository, please confirm that the GitHub group/team that triggered the build has access to the repository.

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[opbld34]

Docs Build status updates of commit d977415:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
	❌Error		Details

• Line 0, Column 0: [Error-git-clone-failed] Failure to clone the repository `https://github.com/Microsoft/templates.docs.msft#master`. This could be caused by an incorrect repository URL, please verify the URL on the Docs Portal (https://ops.microsoft.com). This could also be caused by not having the proper permission the repository, please confirm that the GitHub group/team that triggered the build has access to the repository.

For more details, please refer to the build report.

If you see build warnings/errors with permission issues, it might be due to single sign-on (SSO) enabled on Microsoft's GitHub organizations. Please follow instructions here to re-authorize your GitHub account to Docs Build.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

Note: Your PR may contain errors or warnings unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching in the Docs contributor and Admin Guide
• See the frequently asked questions
• Post your question in the Docs support channel

[opbld32]

Docs Build status updates of commit d977415:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/System.Runtime.InteropServices/SafeHandle.xml	⚠️Warning	View	Details

xml/System.Runtime.InteropServices/SafeHandle.xml

• Line 0, Column 0: [Warning-ECMA2Yaml_UidAssembly_NotMatched] System.Runtime.InteropServices.SafeHandle's moniker netstandard-1.5 can't match any assembly.

For more details, please refer to the build report.

If you see build warnings/errors with permission issues, it might be due to single sign-on (SSO) enabled on Microsoft's GitHub organizations. Please follow instructions here to re-authorize your GitHub account to Docs Build.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

Note: Your PR may contain errors or warnings unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching in the Docs contributor and Admin Guide
• See the frequently asked questions
• Post your question in the Docs support channel