Rollup merge of #134855 - estebank:default-field-values-unstable-docs, r=jieyouxu

Add `default_field_values` entry to unstable book

Tracking issue: rust-lang/rust#132162
RFC: https://github.com/rust-lang/rfcs/blob/master/text/3681-default-field-values.md

Author: matthiaskrgr