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

compiler/rustc_span/src/symbol.rs

@@ -696,6 +696,7 @@ symbols! {
         fabsf32,
         fabsf64,
         fadd_fast,
+        fake_variadic,
         fdiv_fast,
         feature,
         fence,
@@ -1471,7 +1472,6 @@ symbols! {
         tuple,
         tuple_from_req,
         tuple_indexing,
-        tuple_variadic,
         two_phase,
         ty,
         type_alias_enum_variants,