Document heap allocation location guarantee

src/doc/reference.md

@@ -3909,6 +3909,9 @@ The _heap_ is a general term that describes boxes. The lifetime of an
 allocation in the heap depends on the lifetime of the box values pointing to
 it. Since box values may themselves be passed in and out of frames, or stored
 in the heap, heap allocations may outlive the frame they are allocated within.
+An allocation in the heap is guaranteed to reside at a single location in the
+heap for the whole lifetime of the allocation - it will never be relocated as
+a result of moving a box value.
 
 ### Memory ownership