Issue #14954: Clarify the interaction of weak references and garbage collection.

Patch by Ethan Furman.

Doc/library/weakref.rst

@@ -24,7 +24,10 @@ by a weak reference.
 A weak reference to an object is not enough to keep the object alive: when the
 only remaining references to a referent are weak references,
 :term:`garbage collection` is free to destroy the referent and reuse its memory
-for something else.  A primary use for weak references is to implement caches or
+for something else.  However, until the object is actually destroyed the weak
+reference may return the object even if there are no strong references to it.
+
+A primary use for weak references is to implement caches or
 mappings holding large objects, where it's desired that a large object not be
 kept alive solely because it appears in a cache or mapping.
 

Misc/ACKS

@@ -317,6 +317,7 @@ Jim Fulton
 Tadayoshi Funaba
 Gyro Funch
 Peter Funk
+Ethan Furman
 Geoff Furnish
 Ulisses Furquim
 Hagen Fürstenau