Apply suggestions from code review

Co-Authored-By: Ralf Jung <[email protected]>

Author: ssomers

src/liballoc/collections/btree/node.rs

@@ -591,6 +591,7 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         unsafe { &mut *(self.root as *mut Root<K, V>) }
     }
 
+    /// The caller must ensure that the node is not the shared root.
     fn into_key_slice_mut(mut self) -> &'a mut [K] {
         debug_assert!(!self.is_shared_root());
         // We cannot be the shared root, so `as_leaf_mut` is okay.

src/liballoc/collections/btree/search.rs

@@ -62,7 +62,7 @@ where
     // This function is defined over all borrow types (immutable, mutable, owned),
     // and may be called on the shared root in each case.
     // Crucially, we use `keys()` here, i.e., we work with immutable data.
-    // We do not need to make `keys_mut()` public and require support for the shared root.
+    // `keys_mut()` does not support the shared root, so we cannot use it.
     // Using `keys()` is fine here even if BorrowType is mutable, as all we return
     // is an index -- not a reference.
     for (i, k) in node.keys().iter().enumerate() {