Doc: mention --delta-base-offset is the default for Porcelain commands

The underlying pack-objects plumbing command still needs an explicit
option from the command line, but these days Porcelain passes the
option, so there is no need for end users to worry about it anymore.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-objects.txt

@@ -192,13 +192,18 @@ self-contained. Use `git index-pack --fix-thin`
 --delta-base-offset::
 	A packed archive can express the base object of a delta as
 	either a 20-byte object name or as an offset in the
-	stream, but older versions of git don't understand the
+	stream, but ancient versions of git don't understand the
 	latter.  By default, 'git pack-objects' only uses the
 	former format for better compatibility.  This option
 	allows the command to use the latter format for
 	compactness.  Depending on the average delta chain
 	length, this option typically shrinks the resulting
 	packfile by 3-5 per-cent.
++
+Note: Porcelain commands such as `git gc` (see linkgit:git-gc[1]),
+`git repack` (see linkgit:git-repack[1]) pass this option by default
+in modern git when they put objects in your repository into pack files.
+So does `git bundle` (see linkgit:git-bundle[1]) when it creates a bundle.
 
 --threads=<n>::
 	Specifies the number of threads to spawn when searching for best