Add explanation for serializing a DataMember with EmitDefaultValue as false.

[LI-Yanting]

Summary

Explain explicitly that a DataMember with a non default value will still be serialized despite its EmitDefaultValue = false.

[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.

[opbld30]

Docs Build status updates of commit 243873f:

✅ Validation status: passed

File	Status	Preview URL	Details
samples/snippets/csharp/VS_Snippets_CFX/datamemberattribute/cs/overview.cs	✅Succeeded	View

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[adamsitnik]

cc @StephenMolloy @HongGit

[BillWagner]

ping @carlossanlop to route for review.