Document `nested_default_fields` in APIv2

[Curtis Maloney]

Pertinent section of the Wagtail docs

https://docs.wagtail.org/en/stable/advanced_topics/api/index.html

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Details

Whilst the nested_default_fields functionality seems well polished, it's not mentioned anywhere in the documentation.

[ARUP GOPE]

Can I work on it?

[Matt Westcott]

@arup1221 Yes, anyone is free to work on any issue - no need to ask permission.