pack-write/docs: update regarding pack naming

The index-pack documentation explicitly states that the pack
name is derived from the sorted list of object names, but
since commit 1190a1acf8 ("pack-objects: name pack files
after trailer hash") that isn't true anymore.

Be less explicit in the docs as to what the exact output is,
and just say that it's whatever goes into the pack name.

Also update a comment on write_idx_file() since it no longer
modifies the sha1 variable (it's const now anyway), as noted
by Junio.

Fixes: 1190a1acf8 ("pack-objects: name pack files after trailer hash")
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-index-pack.txt

@@ -91,8 +91,8 @@ OPTIONS
 Note
 ----
 
-Once the index has been created, the list of object names is sorted
-and the SHA-1 hash of that list is printed to stdout. If --stdin was
+Once the index has been created, the hash that goes into the name of
+the pack/idx file is printed to stdout. If --stdin was
 also used then this is prefixed by either "pack\t", or "keep\t" if a
 new .keep file was successfully created. This is useful to remove a
 .keep file used as a lock to prevent the race with 'git repack'

pack-write.c

@@ -38,9 +38,8 @@ static int need_large_offset(off_t offset, const struct pack_idx_option *opts)
 }
 
 /*
- * On entry *sha1 contains the pack content SHA1 hash, on exit it is
- * the SHA1 hash of sorted object names. The objects array passed in
- * will be sorted by SHA1 on exit.
+ * The *sha1 contains the pack content SHA1 hash.
+ * The objects array passed in will be sorted by SHA1 on exit.
  */
 const char *write_idx_file(const char *index_name, struct pack_idx_entry **objects,
 			   int nr_objects, const struct pack_idx_option *opts,