Rollup merge of #92264 - Shadlock0133:patch-1, r=the8472

Remove `maybe_uninit_extra` feature from Vec docs

In `Vec`, two doc tests are using `MaybeUninit::write` , stabilized in 1.55. This makes docs' usage of `maybe_uninit_extra` feature unnecessary.

library/alloc/src/vec/mod.rs

@@ -2043,7 +2043,7 @@ pub fn leak<'a>(self) -> &'a mut [T]
     /// # Examples
     ///
     /// ```
-    /// #![feature(vec_spare_capacity, maybe_uninit_extra)]
+    /// #![feature(vec_spare_capacity)]
     ///
     /// // Allocate vector big enough for 10 elements.
     /// let mut v = Vec::with_capacity(10);
@@ -2102,7 +2102,7 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
     /// # Examples
     ///
     /// ```
-    /// #![feature(vec_split_at_spare, maybe_uninit_extra)]
+    /// #![feature(vec_split_at_spare)]
     ///
     /// let mut v = vec![1, 1, 2];
     ///