Clarify the lifetimes of allocations returned by the Allocator trait

The previous definition (accidentally) disallowed the implementation of
stack-based allocators whose memory would become invalid once the
lifetime of the allocator type ended.

This also ensures the validity of the following blanket implementation:
```rust
impl<A: Allocator> Allocator for &'_ A {}
```

Author: Amanieu

library/core/src/alloc/mod.rs

@@ -95,8 +95,10 @@ impl fmt::Display for AllocError {
 /// # Safety
 ///
 /// * 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.
+///   valid memory and retain their validity while they are [*currently allocated*] and the shorter
+///   of:
+///   - the borrow-checker lifetime of the allocator type itself.
+///   - as long as 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