`AdditionalProperties` and `UnparsedObject` are undocumented

[mx-psi]

Describe the bug

All autogenerated structs have an AdditionalProperties and UnparsedObject. AdditionalProperties is used when using MarshalJSON to add any extra fields, but is currently undocumented. I don't know what the purpose of UnparsedObject is. We should document it!

To Reproduce
Steps to reproduce the behavior:

1. Go to https://datadoghq.dev/datadog-api-client-go/pkg/github.com/DataDog/datadog-api-client-go/v2/api/datadogV2/#MetricPayload
2. See that these two fields are undocumented

Expected behavior

All fields have a docstring documenting what they do.

[github-actions]

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