fix comment on Allocator trait

Author: reez12g

library/core/src/alloc/mod.rs

@@ -94,8 +94,9 @@ impl fmt::Display for AllocError {
 ///
 /// # Safety
 ///
-/// * Memory blocks returned from an allocator must point to valid memory and retain their validity
-///   until the instance and all of its copies and clones are dropped,
+/// * Memory blocks returned from an allocator that are [*currently allocated*] must point to
+///   valid memory and retain their validity while they are [*currently allocated*] and at
+///   least one of the instance and all of its clones has not been dropped.
 ///
 /// * copying, cloning, or moving the allocator must not invalidate memory blocks returned from this
 ///   allocator. A copied or cloned allocator must behave like the same allocator, and