docs: add note about GFS (#6006)

docs: add note about grandfather-father-son backup retention policy / rotation scheme

src/borg/archiver.py

@@ -3905,6 +3905,9 @@ class Archiver:
         The prune command prunes a repository by deleting all archives not matching
         any of the specified retention options. This command is normally used by
         automated backup scripts wanting to keep a certain number of historic backups.
+        This retention policy is commonly referred to as
+        `GFS <https://en.wikipedia.org/wiki/Backup_rotation_scheme#Grandfather-father-son>`_
+        (Grandfather-father-son) backup rotation scheme.
 
         Also, prune automatically removes checkpoint archives (incomplete archives left
         behind by interrupted backup runs) except if the checkpoint is the latest