Rollup merge of rust-lang#146887 - taiki-e:rc-doc-feature, r=joboet

Remove unused #![feature(get_mut_unchecked)] in Rc and Arc examples

rust-lang#93109 removed the use of APIs enabled by this feature in these examples, but the `#![feature]` attributes ware not removed.

Author: GuillaumeGomez

library/alloc/src/rc.rs

@@ -480,8 +480,6 @@ impl<T> Rc<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::rc::Rc;
     ///
     /// let mut five = Rc::<u32>::new_uninit();
@@ -572,7 +570,6 @@ impl<T> Rc<T> {
     ///
     /// ```
     /// #![feature(allocator_api)]
-    /// #![feature(get_mut_unchecked)]
     ///
     /// use std::rc::Rc;
     ///
@@ -1014,8 +1011,6 @@ impl<T> Rc<[T]> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::rc::Rc;
     ///
     /// let mut values = Rc::<[u32]>::new_uninit_slice(3);
@@ -1181,8 +1176,6 @@ impl<T, A: Allocator> Rc<mem::MaybeUninit<T>, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::rc::Rc;
     ///
     /// let mut five = Rc::<u32>::new_uninit();
@@ -1218,8 +1211,6 @@ impl<T, A: Allocator> Rc<[mem::MaybeUninit<T>], A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::rc::Rc;
     ///
     /// let mut values = Rc::<[u32]>::new_uninit_slice(3);

library/alloc/src/sync.rs

@@ -480,8 +480,6 @@ impl<T> Arc<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::sync::Arc;
     ///
     /// let mut five = Arc::<u32>::new_uninit();
@@ -586,7 +584,6 @@ impl<T> Arc<T> {
     ///
     /// ```
     /// #![feature(allocator_api)]
-    /// #![feature(get_mut_unchecked)]
     ///
     /// use std::sync::Arc;
     ///
@@ -1156,8 +1153,6 @@ impl<T> Arc<[T]> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::sync::Arc;
     ///
     /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
@@ -1326,8 +1321,6 @@ impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::sync::Arc;
     ///
     /// let mut five = Arc::<u32>::new_uninit();
@@ -1364,8 +1357,6 @@ impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(get_mut_unchecked)]
-    ///
     /// use std::sync::Arc;
     ///
     /// let mut values = Arc::<[u32]>::new_uninit_slice(3);