Clarify repack -n documentation

While repacking a local repository a coworker thought the -n option
was necessary to git-repack to keep it from updating some unknown
file on the central server we all share.  Explaining further what
the option is (not) doing helps to make it clear the option does
not impact any remote repositories the user may have configured.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-repack.txt

@@ -55,8 +55,11 @@ OPTIONS
 	linkgit:git-pack-objects[1].
 
 -n::
-        Do not update the server information with
-        `git update-server-info`.
+	Do not update the server information with
+	`git update-server-info`.  This option skips
+	updating local catalog files needed to publish
+	this repository (or a direct copy of it)
+	over HTTP or FTP.  See gitlink:git-update-server-info[1].
 
 --window=[N], --depth=[N]::
 	These two options affect how the objects contained in the pack are