Improve memory-model section very slightly.

Author: graydon

src/doc/reference.md

@@ -3842,8 +3842,10 @@ is to be destroyed.
 # Memory model
 
 A Rust program's memory consists of a static set of *items* and a *heap*.
-Immutable portions of the heap may be shared between threads, mutable portions
-may not.
+Immutable portions of the heap may be safely shared between threads, mutable
+portions may not be safely shared, but several mechanisms for effectively-safe
+sharing of mutable values, built on unsafe code but enforcing a safe locking
+discipline, exist in the standard library.
 
 Allocations in the stack consist of *variables*, and allocations in the heap
 consist of *boxes*.