Add table headers for accessibility

[gewarren]

Contributes to #9813.

(Hide whitespace changes.)

[learn-build-service-prod]

Learn Build status updates of commit a87be4a:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.AddIn.Hosting/AddInProcess.xml	✅Succeeded	View
xml/System.AddIn.Hosting/Platform.xml	✅Succeeded	View
xml/System.Buffers.Text/Utf8Formatter.xml	✅Succeeded	View
xml/System.Buffers.Text/Utf8Parser.xml	✅Succeeded	View
xml/System.Collections.Concurrent/ConcurrentDictionary`2.xml	✅Succeeded	View
xml/System.Collections.Immutable/ImmutableArray`1.xml	✅Succeeded	View
xml/System.ComponentModel/DesignerProperties.xml	✅Succeeded	View
xml/System.Data.SqlClient/SqlDataReader.xml	✅Succeeded	View
xml/System.IO.Packaging/PackageDigitalSignature.xml	✅Succeeded	View
xml/System.IO.Packaging/PackageRelationship.xml	✅Succeeded	View
xml/System.Numerics/BigInteger.xml	✅Succeeded	View
xml/System.Security.Cryptography/PaddingMode.xml	✅Succeeded	View
xml/System.Text.RegularExpressions/Match.xml	✅Succeeded	View
xml/System.Text.RegularExpressions/Regex.xml	✅Succeeded	View
xml/System.Web.Security.AntiXss/AntiXssEncoder.xml	✅Succeeded	View
xml/System.Web.Security/ActiveDirectoryMembershipUser.xml	✅Succeeded	View
xml/System.Web.UI.DataVisualization.Charting/DataPoint.xml	✅Succeeded	View
xml/System.Web.UI.DataVisualization.Charting/DataPointCollection.xml	✅Succeeded	View
xml/System.Web.UI.WebControls/CheckBoxList.xml	✅Succeeded	View
xml/System.Web.UI.WebControls/DataList.xml	✅Succeeded	View
xml/System.Web.UI.WebControls/Parameter.xml	✅Succeeded	View
xml/System.Web.UI.WebControls/RadioButtonList.xml	✅Succeeded	View
xml/System.Web.UI.WebControls/RepeatDirection.xml	✅Succeeded	View
xml/System.Web.UI.WebControls/RepeatInfo.xml	✅Succeeded	View
xml/System.Web.UI/ChtmlTextWriter.xml	✅Succeeded	View

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[BillWagner]

This also LGTM @gewarren

Let's