rustdoc: show tuple impls as impl Trait for (T, ...)

This commit adds a new unstable attribute, `#[doc(tuple_varadic)]`, that
shows a 1-tuple as `(T, ...)` instead of just `(T,)`, and links to a section
in the tuple primitive docs that talks about these.

compiler/rustc_span/src/symbol.rs

@@ -1439,6 +1439,7 @@ symbols! {
         tuple,
         tuple_from_req,
         tuple_indexing,
+        tuple_varadic,
         two_phase,
         ty,
         type_alias_enum_variants,