Documentation: pack.compression: explain how to recompress

Add a small remark about how to recompress all existing objects after
changing the compression level for pack files.

Signed-off-by: Jan Krüger <jk@jk.gs>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1466,6 +1466,10 @@ pack.compression::
 	not set,  defaults to -1, the zlib default, which is "a default
 	compromise between speed and compression (currently equivalent
 	to level 6)."
++
+Note that changing the compression level will not automatically recompress
+all existing objects. You can force recompression by passing the -F option
+to linkgit:git-repack[1].
 
 pack.deltaCacheSize::
 	The maximum memory in bytes used for caching deltas in