Rollup merge of rust-lang#120266 - steffahn:a_rc_into_inner_docs, r=Mark-Simulacrum

Improve documentation for [A]Rc::into_inner

General improvements, and also aims to better encourage the reader to actually check out Arc::try_unwrap.

This addresses concerns from rust-lang#106894 (comment).

Rendered:

![Screenshot_20240123_114436](https://github.com/rust-lang/rust/assets/3986214/68896d62-13e0-4f3a-8073-91d8e77c5554)
![Screenshot_20240123_114455](https://github.com/rust-lang/rust/assets/3986214/dc58e4bd-dd7f-40b1-bc50-fd6200dde593)

Author: matthiaskrgr

library/alloc/src/rc.rs

@@ -939,8 +939,11 @@ impl<T, A: Allocator> Rc<T, A> {
     /// it is guaranteed that exactly one of the calls returns the inner value.
     /// This means in particular that the inner value is not dropped.
     ///
-    /// This is equivalent to `Rc::try_unwrap(this).ok()`. (Note that these are not equivalent for
-    /// [`Arc`](crate::sync::Arc), due to race conditions that do not apply to `Rc`.)
+    /// [`Rc::try_unwrap`] is conceptually similar to `Rc::into_inner`.
+    /// And while they are meant for different use-cases, `Rc::into_inner(this)`
+    /// is in fact equivalent to <code>[Rc::try_unwrap]\(this).[ok][Result::ok]()</code>.
+    /// (Note that the same kind of equivalence does **not** hold true for
+    /// [`Arc`](crate::sync::Arc), due to race conditions that do not apply to `Rc`!)
     #[inline]
     #[stable(feature = "rc_into_inner", since = "1.70.0")]
     pub fn into_inner(this: Self) -> Option<T> {

library/alloc/src/sync.rs

@@ -983,9 +983,13 @@ impl<T, A: Allocator> Arc<T, A> {
     /// it is guaranteed that exactly one of the calls returns the inner value.
     /// This means in particular that the inner value is not dropped.
     ///
-    /// The similar expression `Arc::try_unwrap(this).ok()` does not
-    /// offer such a guarantee. See the last example below
-    /// and the documentation of [`Arc::try_unwrap`].
+    /// [`Arc::try_unwrap`] is conceptually similar to `Arc::into_inner`, but it
+    /// is meant for different use-cases. If used as a direct replacement
+    /// for `Arc::into_inner` anyway, such as with the expression
+    /// <code>[Arc::try_unwrap]\(this).[ok][Result::ok]()</code>, then it does
+    /// **not** give the same guarantee as described in the previous paragraph.
+    /// For more information, see the examples below and read the documentation
+    /// of [`Arc::try_unwrap`].
     ///
     /// # Examples
     ///