docs: add note about GFS (#6006)

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

src/borg/archiver.py

@@ -4309,7 +4309,9 @@ def define_borg_mount(parser):
         Important: Repository disk space is **not** freed until you run ``borg compact``.
 
         This command is normally used by automated backup scripts wanting to keep a
-        certain number of historic backups.
+        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