docs: improve changelog entry for `git pack-refs --auto`

The changelog entry for the new `git pack-refs --auto` mode only says
that the new flag is useful, but doesn't really say what it does. Add
some more information.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/RelNotes/2.45.0.txt b/Documentation/RelNotes/2.45.0.txt
index 3845866..0b12209 100644
--- a/Documentation/RelNotes/2.45.0.txt
+++ b/Documentation/RelNotes/2.45.0.txt
@@ -77,8 +77,10 @@
    skip showing the hunk immediately after it has already been shown, and
    an additional action to explicitly ask to reshow the current hunk.
 
- * "git pack-refs" learned the "--auto" option, which is a useful
-   addition to be triggered from "git gc --auto".
+ * "git pack-refs" learned the "--auto" option, which defers the decision of
+   whether and how to pack to the ref backend. This is used by the reftable
+   backend to avoid repacking of an already-optimal ref database. The new mode
+   is triggered from "git gc --auto".
 
  * "git add -u <pathspec>" and "git commit [-i] <pathspec>" did not
    diagnose a pathspec element that did not match any files in certain