Fix internal rustdoc broken links

As it was suggested in rust-lang#81037 `SpecFromIter` is not
in the scope and therefore (even it should fail),
we get a warning when we try do document private
intems in `rust/library/alloc/`.

This fixes rust-lang#81037 by adding the trait in the scope
and also adding an `allow(unused_imports)` flag so that
the compiler does not complain, Since the trait is not used
per se in the code, it's just needed to have properly documented
docs.

Author: CPerezz

alloc/src/vec/spec_from_iter_nested.rs

@@ -1,6 +1,8 @@
 use core::iter::TrustedLen;
 use core::ptr::{self};
 
+#[allow(unused_imports)]
+use super::SpecFromIter;
 use super::{SpecExtend, Vec};
 
 /// Another specialization trait for Vec::from_iter