Add note about allowing out of order metadata properties

[gewarren]

Contributes to dotnet/docs#42592.

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Tagging subscribers to this area: @dotnet/area-system-text-json

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[eiriktsarpalis]

Would it make sense to also include mention of this feature in the polymorphism document?

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0

Polymorphism is the primary motivator for introducing this feature (e.g. because you're reading data off an external API that happens to place the $type discriminator in the middle of an object.

[BillWagner]

This LGTM @gewarren

Let's

[gewarren]

Would it make sense to also include mention of this feature in the polymorphism document?

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0

Polymorphism is the primary motivator for introducing this feature (e.g. because you're reading data off an external API that happens to place the $type discriminator in the middle of an object.

Yes, I will add it there separately too.