Update section on untracking

Author: markshannon

InternalDocs/garbage_collector.md

@@ -520,8 +520,8 @@ of `PyGC_Head` discussed in the `Memory layout and object structure`_ section:
   currently in.  Instead, when that's needed, ad hoc tricks (like the
   `NEXT_MASK_UNREACHABLE` flag) are employed.
 
-Optimization: delay tracking containers
-=======================================
+Optimization: delayed untracking containers
+===========================================
 
 Certain types of containers cannot participate in a reference cycle, and so do
 not need to be tracked by the garbage collector. Untracking these objects
@@ -536,8 +536,8 @@ a container:
 As a general rule, instances of atomic types aren't tracked and instances of
 non-atomic types (containers, user-defined objects...) are.  However, some
 type-specific optimizations can be present in order to suppress the garbage
-collector footprint of simple instances. Some examples of native types that
-benefit from delayed tracking:
+collector footprint of simple instances. Historically, both dictionaries and
+tuples were untracked during garbage collection. Now it is only tuples:
 
 - Tuples containing only immutable objects (integers, strings etc,
   and recursively, tuples of immutable objects) do not need to be tracked. The
@@ -546,14 +546,8 @@ benefit from delayed tracking:
   tuples at creation time. Instead, all tuples except the empty tuple are tracked
   when created. During garbage collection it is determined whether any surviving
   tuples can be untracked. A tuple can be untracked if all of its contents are
-  already not tracked. Tuples are examined for untracking in all garbage collection
-  cycles. It may take more than one cycle to untrack a tuple.
-
-- Dictionaries containing only immutable objects also do not need to be tracked.
-  Dictionaries are untracked when created. If a tracked item is inserted into a
-  dictionary (either as a key or value), the dictionary becomes tracked. During a
-  full garbage collection (all generations), the collector will untrack any dictionaries
-  whose contents are not tracked.
+  already not tracked. Tuples are examined for untracking when moved from the
+  young to the old generation.
 
 The garbage collector module provides the Python function `is_tracked(obj)`, which returns
 the current tracking status of the object. Subsequent garbage collections may change the
@@ -566,11 +560,9 @@ tracking status of the object.
       False
       >>> gc.is_tracked([])
       True
-      >>> gc.is_tracked({})
+      >>> gc.is_tracked(("a": 1))
       False
       >>> gc.is_tracked({"a": 1})
-      False
-      >>> gc.is_tracked({"a": []})
       True
 ```