Update git-unpack-objects documentation.

Document that git-unpack-objects will not produce any
results when used on a pack that exists in a repository;
move it first.

Signed-off-by: Sean Estabrooks <seanlkml@sympatico.ca>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-unpack-objects.txt

@@ -13,9 +13,16 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Reads a packed archive (.pack) from the standard input, and
-expands the objects contained in the pack into "one-file
-one-object" format in $GIT_OBJECT_DIRECTORY.
+Read a packed archive (.pack) from the standard input, expanding
+the objects contained within and writing them into the repository in
+"loose" (one object per file) format.
+
+Objects that already exist in the repository will *not* be unpacked
+from the pack-file.  Therefore, nothing will be unpacked if you use
+this command on a pack-file that exists within the target repository.
+
+Please see the `git-repack` documentation for options to generate
+new packs and replace existing ones.
 
 OPTIONS
 -------