Add docs missing for parameterless ctors in M.E.Configuration

[maryamariyan]

Contributes to dotnet/runtime#43869

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[tarekgh]

LGTM. Thanks @maryamariyan

[opbld33]

Learn Build status updates of commit 71262df:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/Microsoft.Extensions.Configuration.CommandLine/CommandLineConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.EnvironmentVariables/EnvironmentVariablesConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Ini/IniConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Ini/IniStreamConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Json/JsonConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Json/JsonStreamConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Memory/MemoryConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.NewtonsoftJson/NewtonsoftJsonConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.NewtonsoftJson/NewtonsoftJsonStreamConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.UserSecrets/PathHelper.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Xml/XmlConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration.Xml/XmlStreamConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration/ChainedConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration/ConfigurationKeyComparer.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration/ConfigurationReloadToken.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration/FileConfigurationSource.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration/FileLoadExceptionContext.xml	✅Succeeded	View
xml/Microsoft.Extensions.Configuration/StreamConfigurationSource.xml	✅Succeeded	View

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