bpo-37051: Refine note on what objects are hashable (GH-13587)

Doc/glossary.rst

@@ -512,8 +512,10 @@ Glossary
       Hashability makes an object usable as a dictionary key and a set member,
       because these data structures use the hash value internally.
 
-      All of Python's immutable built-in objects are hashable; mutable
-      containers (such as lists or dictionaries) are not.  Objects which are
+      Most of Python's immutable built-in objects are hashable; mutable
+      containers (such as lists or dictionaries) are not; immutable
+      containers (such as tuples and frozensets) are only hashable if
+      their elements are hashable.  Objects which are
       instances of user-defined classes are hashable by default.  They all
       compare unequal (except with themselves), and their hash value is derived
       from their :func:`id`.