bpo-41455: Provide a link to how the third generation is collected in the GC docs (GH-21703) (GH-21788)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
(cherry picked from commit 82ca8fa)

Co-authored-by: Yaroslav Pankovych <31005942+P-Alban@users.noreply.github.com>

Author: miss-islington

Doc/library/gc.rst

@@ -103,9 +103,9 @@ The :mod:`gc` module provides the following functions:
    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.  Similarly, *threshold2*
-   controls the number of collections of generation ``1`` before collecting
-   generation ``2``.
+   examined, then generation ``1`` is examined as well.
+   With the third generation, things are a bit more complicated,
+   see `Collecting the oldest generation <https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation>`_ for more information.
 
 
 .. function:: get_count()