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>

Author: nelhage

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
-	ignoring all packed objects, it only ignores objects
-	that are packed and/or not in the local object store
-	(i.e. borrowed from an alternate).
+	This flag causes an object that is borrowed from an alternate
+	object store to be ignored even if it appears in the standard
+	input.
 
 --non-empty::
         Only create a packed archive if it would contain at