Auto merge of #98180 - notriddle:notriddle/rustdoc-fn, r=petrochenkov,GuillaumeGomez

Improve the function pointer docs

This is #97842 but for function pointers instead of tuples. The concept is basically the same.

* Reduce duplicate impls; show `fn (T₁, T₂, …, Tₙ)` and include a sentence saying that there exists up to twelve of them.
* Show `Copy` and `Clone`.
* Show auto traits like `Send` and `Sync`, and blanket impls like `Any`.

https://notriddle.com/notriddle-rustdoc-test/std/primitive.fn.html

Author: bors