Documentation: pack-objects: Clarify --local's semantics.

The current documentation suggests that --local also ignores any
objects in local packs, which is incorrect. Change the language to be
clearer and more parallel to the other options that ignore objects.

While we're at it, fix a trivial error in --incremental's
documentation.

Signed-off-by: Nelson Elhage <nelhage@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-objects.txt

@@ -119,14 +119,13 @@ base-name::
 	standard input.
 
 --incremental::
-	This flag causes an object already in a pack ignored
+	This flag causes an object already in a pack to be ignored
 	even if it appears in the standard input.
 
 --local::
-	This flag is similar to `--incremental`; instead of
+	This flag causes an object that is borrowed from an alternate
-	ignoring all packed objects, it only ignores objects
+	object store to be ignored even if it appears in the standard
-	that are packed and/or not in the local object store
+	input.
-	(i.e. borrowed from an alternate).
 
 --non-empty::
         Only create a packed archive if it would contain at