Rollup merge of rust-lang#134662 - ionicmc-rs:any-safety-docs, r=Amanieu

Fix safety docs for `dyn Any + Send {+ Sync}`

Fixes the `# Safety` docs for `dyn Any + Send`'s `downcast_{mut/ref}_unchecked` to show the direct instructions , where previously the would tell the user to find the docs on `dyn Any` themselves.

This also adds them for `downcast_{mut/ref}_unchecked` on `dyn Any + Send + Sync`

Author: Zalathar

library/core/src/any.rs

@@ -423,7 +423,8 @@ impl dyn Any + Send {
     ///
     /// # Safety
     ///
-    /// Same as the method on the type `dyn Any`.
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
     #[unstable(feature = "downcast_unchecked", issue = "90850")]
     #[inline]
     pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
@@ -451,7 +452,8 @@ impl dyn Any + Send {
     ///
     /// # Safety
     ///
-    /// Same as the method on the type `dyn Any`.
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
     #[unstable(feature = "downcast_unchecked", issue = "90850")]
     #[inline]
     pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
@@ -552,6 +554,10 @@ impl dyn Any + Send + Sync {
     ///     assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
     /// }
     /// ```
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
     #[unstable(feature = "downcast_unchecked", issue = "90850")]
     #[inline]
     pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
@@ -576,6 +582,10 @@ impl dyn Any + Send + Sync {
     ///
     /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
     /// ```
+    /// # Safety
+    ///
+    /// The contained value must be of type `T`. Calling this method
+    /// with the incorrect type is *undefined behavior*.
     #[unstable(feature = "downcast_unchecked", issue = "90850")]
     #[inline]
     pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {