Rollup merge of #81497 - camelid:rustdoc-display_fn-remove-cell, r=jyn514
Rollup merge of #81497 - camelid:rustdoc-display_fn-remove-cell, r=jyn514

rustdoc: Move `display_fn` struct inside `display_fn`

This makes it clear that it's an implementation detail of `display_fn`
and shouldn't be used elsewhere, and it enforces in the compiler that no
one else can use it.

r? ````@GuillaumeGomez````

aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust