Documentation/git-repack.txt: document new -A behaviour

Add paragraph for the -A option, and describe the new behaviour
that makes unreachable objects loose.
Signed-off-by: NChris Frey <cdfrey@foursquare.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-repack.txt

@@ -8,7 +8,7 @@ git-repack - Pack unpacked objects in a repository
 
 SYNOPSIS
 --------
-'git-repack' [-a] [-d] [-f] [-l] [-n] [-q] [--window=N] [--depth=N]
+'git-repack' [-a] [-A] [-d] [-f] [-l] [-n] [-q] [--window=N] [--depth=N]
 
 DESCRIPTION
 -----------
@@ -37,6 +37,18 @@ OPTIONS
 	leaves behind, but `git fsck --full` shows as
 	dangling.
 
+-A::
+	Same as `-a`, but any unreachable objects in a previous
+	pack become loose, unpacked objects, instead of being
+	left in the old pack.  Unreachable objects are never
+	intentionally added to a pack, even when repacking.
+	When used with '-d', this option
+	prevents unreachable objects from being immediately
+	deleted by way of being left in the old pack and then
+	removed.  Instead, the loose unreachable objects
+	will be pruned according to normal expiry rules
+	with the next linkgit:git-gc[1].
+
 -d::
 	After packing, if the newly created packs make some
 	existing packs redundant, remove the redundant packs.