Rollup merge of rust-lang#76062 - pickfire:patch-13, r=jyn514

Vec slice example fix style and show type elision

library/alloc/src/vec.rs

@@ -174,7 +174,9 @@ use crate::raw_vec::RawVec;
 ///
 /// // ... and that's all!
 /// // you can also do it like this:
-/// let x : &[usize] = &v;
+/// let u: &[usize] = &v;
+/// // or like this:
+/// let u: &[_] = &v;
 /// ```
 ///
 /// In Rust, it's more common to pass slices as arguments rather than vectors