Update GC docs.

Author: nascheme

Doc/library/gc.rst

@@ -128,6 +128,11 @@ The :mod:`gc` module provides the following functions:
    starts. For each collection, all the objects in the young generation and some
    fraction of the old generation is collected.
 
+   In the free-threaded build, the increase in process resident set size (RSS)
+   is also checked before running the collector.  If the RSS has not increased
+   by 10% since the last collection and the net number of object allocations
+   has not exceeded 40 times *threshold0*, the collection is not run.
+
    The fraction of the old generation that is collected is **inversely** proportional
    to *threshold1*. The larger *threshold1* is, the slower objects in the old generation
    are collected.