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

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Author: ypankovych

Doc/library/gc.rst

@@ -106,9 +106,9 @@ 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()