Add missing 7.0 docs for isolated APIs in various namespaces

[carlossanlop]

Found a few scattered APIs that were missing some documentation.

Area owners, please review:

• @dotnet/area-system-text-json
• @dotnet/area-system-text-regularexpressions
• @kouvel (for System.Threading)
• @dotnet/area-system-runtime (for System.Timers)
• @dotnet/area-extensions-configuration
• @dotnet/area-system-componentmodel
• @dotnet/area-system-io (for System.Formats.Tar)

[tarekgh]

Microsoft.Extensions and System.ComponentModel changes LGTM.

[eiriktsarpalis]

System.Text.Json changes look good to me.

[opbld34]

Docs Build status updates of commit db75935:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/Microsoft.Extensions.Configuration/ConfigurationDebugViewContext.xml	⚠️Warning	View	Details
xml/System.Text.RegularExpressions/RegexRunner.xml	⚠️Warning	View	Details
xml/Microsoft.Extensions.Configuration/ConfigurationManager.xml	✅Succeeded	View
xml/ns-System.Formats.Tar.xml	✅Succeeded	View
xml/System.ComponentModel.Design.Serialization/MemberRelationship.xml	✅Succeeded	View
xml/System.Text.Json.Serialization.Metadata/JsonMetadataServices.xml	✅Succeeded	View
xml/System.Text.Json.Serialization/JsonPolymorphicAttribute.xml	✅Succeeded	View
xml/System.Threading/PeriodicTimer.xml	✅Succeeded	View
xml/System.Timers/Timer.xml	✅Succeeded	View

xml/Microsoft.Extensions.Configuration/ConfigurationDebugViewContext.xml

• Line 0, Column 0: [Warning: xref-not-found] Cross reference not found: 'IConfigurationProvider'.

xml/System.Text.RegularExpressions/RegexRunner.xml

• Line 0, Column 0: [Warning: ECMA2Yaml_CrefTypePrefixMissing] Invalid cref format (NotSupportedException) detected in C:\LocalRun\W\wj1h-s\xml\System.Text.RegularExpressions\RegexRunner.xml
• Line 0, Column 0: [Warning: xref-not-found] Cross reference not found: 'NotSupportedException'.

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