Update GetAccessControl/SetAccessControl docs

[gewarren]

Fixes #9865.

(Hide whitespace changes in diff.)

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Tagging subscribers to this area: @dotnet/area-system-io

[learn-build-service-prod]

Learn Build status updates of commit 812d957:

✅ Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/System.IO/Directory/GetAccessControl/Project.csproj	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System.IO/Directory/GetAccessControl/sample.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System.IO/DirectoryInfo/GetAccessControl/Project.csproj	✅Succeeded
snippets/csharp/System.IO/DirectoryInfo/GetAccessControl/sample.cs	✅Succeeded	View
snippets/csharp/System.IO/File/GetAccessControl/sample.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System.IO/FileInfo/GetAccessControl/Project.csproj	✅Succeeded
snippets/csharp/System.IO/FileInfo/GetAccessControl/sample.cs	✅Succeeded	View
snippets/csharp/System.IO/FileSystemAclExtensions/GetAccessControl/Project.csproj	✅Succeeded
snippets/csharp/System.IO/FileSystemAclExtensions/GetAccessControl/sample.cs	✅Succeeded	View
snippets/fsharp/System.IO/Directory/GetAccessControl/fs.fsproj	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System.IO/Directory/GetAccessControl/sample.fs	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System.IO/DirectoryInfo/GetAccessControl/fs.fsproj	✅Succeeded
snippets/fsharp/System.IO/DirectoryInfo/GetAccessControl/sample.fs	✅Succeeded	View
snippets/visualbasic/System.IO/FileSystemAclExtensions/GetAccessControl/Project.vbproj	✅Succeeded
snippets/visualbasic/System.IO/FileSystemAclExtensions/GetAccessControl/sample.vb	✅Succeeded	View
snippets/visualbasic/VS_Snippets_CLR/IO.File.GetAccessControl-SetAccessControl/VB/sample.vb	✅Succeeded	n/a (file deleted or renamed)
xml/System.IO/Directory.xml	✅Succeeded	View
xml/System.IO/DirectoryInfo.xml	✅Succeeded	View
xml/System.IO/File.xml	✅Succeeded	View
xml/System.IO/FileInfo.xml	✅Succeeded	View
xml/System.IO/FileStream.xml	✅Succeeded	View
xml/System.Security.AccessControl/DirectorySecurity.xml	✅Succeeded	View
xml/System.Security.AccessControl/FileSecurity.xml	✅Succeeded	View
xml/System.Security.AccessControl/FileSystemAccessRule.xml	✅Succeeded	View
xml/System.Security.AccessControl/FileSystemRights.xml	✅Succeeded	View

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[BillWagner]

This LGTM @gewarren

Once the product team is happy, this is ready to merge.