Rollup merge of #45461 - wesleywiser:intrinsics_docs, r=dtolnay

Two small enhancements to intrinsics docs

src/libcore/intrinsics.rs

@@ -676,6 +676,10 @@ extern "rust-intrinsic" {
     pub fn min_align_of<T>() -> usize;
     pub fn pref_align_of<T>() -> usize;
 
+    /// The size of the referenced value in bytes.
+    ///
+    /// The stabilized version of this intrinsic is
+    /// [`std::mem::size_of_val`](../../std/mem/fn.size_of_val.html).
     pub fn size_of_val<T: ?Sized>(_: &T) -> usize;
     pub fn min_align_of_val<T: ?Sized>(_: &T) -> usize;
 
@@ -921,6 +925,9 @@ extern "rust-intrinsic" {
     ///
     /// If the actual type neither requires drop glue nor implements
     /// `Copy`, then may return `true` or `false`.
+    ///
+    /// The stabilized version of this intrinsic is
+    /// [`std::mem::needs_drop`](../../std/mem/fn.needs_drop.html).
     pub fn needs_drop<T>() -> bool;
 
     /// Calculates the offset from a pointer.