Update what's new with the latest GC improvements (GH-17708)

Author: pablogsal

Doc/whatsnew/3.9.rst

@@ -159,6 +159,14 @@ Added constants :data:`~fcntl.F_OFD_GETLK`, :data:`~fcntl.F_OFD_SETLK`
 and :data:`~fcntl.F_OFD_SETLKW`.
 (Contributed by Dong-hee Na in :issue:`38602`.)
 
+gc
+--
+
+When the garbage collector makes a collection in which some objects resurrect
+(they are reachable from outside the isolated cycles after the finalizers have
+been executed), do not block the collection of all objects that are still
+unreachable. (Contributed by Pablo Galindo and Tim Peters in :issue:`38379`.)
+
 os
 --