Auto merge of #97842 - notriddle:notriddle/tuple-docs, r=jsha,GuillaumeGomez

Improve the tuple and unit trait docs

* Reduce duplicate impls; show only the `(T,)` and include a sentence saying that there exists ones up to twelve of them.
* Show `Copy` and `Clone`.
* Show auto traits like `Send` and `Sync`, and blanket impls like `Any`.

Here's the new version:

* <https://notriddle.com/notriddle-rustdoc-test/std/primitive.tuple.html>
* <https://notriddle.com/notriddle-rustdoc-test/std/primitive.unit.html>

compiler/rustc_span/src/symbol.rs

@@ -1449,6 +1449,7 @@ symbols! {
         tuple,
         tuple_from_req,
         tuple_indexing,
+        tuple_variadic,
         two_phase,
         ty,
         type_alias_enum_variants,