Rollup merge of #140121 - blyxyas:code_stats_pub_docs, r=jieyouxu

Document why CodeStats::type_sizes is public

As indicated in [this comment](rust-lang/rust#139876 (comment)) from #139876
> Need some comment, otherwise this pub can be eventually removed as unused.

r? `@nnethercote`

Author: ChrisDenton