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Rollup merge of #134855 - estebank:default-field-values-unstable-docs, r=jieyouxu
Add `default_field_values` entry to unstable book
Tracking issue: https://github.com/rust-lang/rust/issues/132162
RFC: https://github.com/rust-lang/rfcs/blob/master/text/3681-default-field-values.md
