gh-122862: Fix description of the GC allocation counter in set_threshold() docs

Doc/library/gc.rst

@@ -124,12 +124,13 @@ The :mod:`!gc` module provides the following functions:
    generation (generation ``0``).  If an object survives a collection it is moved
    into the next older generation.  Since generation ``2`` is the oldest
    generation, objects in that generation remain there after a collection.  In
-   order to decide when to run, the collector keeps track of the number object
-   allocations and deallocations since the last collection.  When the number of
-   allocations minus the number of deallocations exceeds *threshold0*, collection
-   starts.  Initially only generation ``0`` is examined.  If generation ``0`` has
-   been examined more than *threshold1* times since generation ``1`` has been
-   examined, then generation ``1`` is examined as well.
+   order to decide when to run, the collector keeps a counter that is reset by
+   each collection, incremented by each object allocation, and decremented by
+   each object deallocation, but never below zero.  When this counter exceeds
+   *threshold0*, collection starts.  Initially only generation ``0`` is
+   examined.  If generation ``0`` has been examined more than *threshold1* times
+   since generation ``1`` has been examined, then generation ``1`` is examined
+   as well.
    With the third generation, things are a bit more complicated,
    see `Collecting the oldest generation <https://github.com/python/cpython/blob/ff0ef0a54bef26fc507fbf9b7a6009eb7d3f17f5/InternalDocs/garbage_collector.md#collecting-the-oldest-generation>`_ for more information.