for_each_*_object: give more comprehensive docstrings

We already mention the local/alternate behavior of these
functions, but we can help clarify a few other behaviors:

 - there's no need to mention LOCAL_ONLY specifically, since
   we already reference the flags by type (and as we add
   more flags, we don't want to have to mention each)

 - clarify that reachability doesn't matter here; this is
   all accessible objects

 - what ordering/uniqueness guarantees we give

 - how pack-specific flags are handled for the loose case

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1636,9 +1636,11 @@ enum for_each_object_flags {
 };
 
 /*
- * Iterate over loose objects in both the local
- * repository and any alternates repositories (unless the
- * LOCAL_ONLY flag is set).
+ * Iterate over all accessible loose objects without respect to
+ * reachability. By default, this includes both local and alternate objects.
+ * The order in which objects are visited is unspecified.
+ *
+ * Any flags specific to packs are ignored.
  */
 int for_each_loose_object(each_loose_object_fn, void *,
 			  enum for_each_object_flags flags);

packfile.h

@@ -149,10 +149,14 @@ extern int has_object_pack(const struct object_id *oid);
 extern int has_pack_index(const unsigned char *sha1);
 
 /*
- * Iterate over packed objects in both the local
- * repository and any alternates repositories (unless the
- * FOR_EACH_OBJECT_LOCAL_ONLY flag is set). See cache.h for the complete list
- * of flags.
+ * Iterate over all accessible packed objects without respect to reachability.
+ * By default, this includes both local and alternate packs.
+ *
+ * Note that some objects may appear twice if they are found in multiple packs.
+ * Each pack is visited in an unspecified order. Objects within a pack are
+ * visited in pack-idx order (i.e., sorted by oid).
+ *
+ * The list of flags can be found in cache.h.
  */
 typedef int each_packed_object_fn(const struct object_id *oid,
 				  struct packed_git *pack,