update-index: there no longer is apply --index-info

Back when we removed `git apply --index-info` in 2007, we forgot to
adjust the documentation for update-index that reads its output.

Let's reorder the description of three formats to present the other
two formats that are still generated by git commands before this
format, and stop mentioning `git apply --index-info`.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -268,23 +268,20 @@ USING --INDEX-INFO
 multiple entry definitions from the standard input, and designed
 specifically for scripts.  It can take inputs of three formats:
 
-    . mode         SP sha1          TAB path
-+
-The first format is what "git-apply --index-info"
-reports, and used to reconstruct a partial tree
-that is used for phony merge base tree when falling
-back on 3-way merge.
-
     . mode SP type SP sha1          TAB path
 +
-The second format is to stuff 'git ls-tree' output
-into the index file.
+This format is to stuff `git ls-tree` output into the index.
 
     . mode         SP sha1 SP stage TAB path
 +
 This format is to put higher order stages into the
 index file and matches 'git ls-files --stage' output.
 
+    . mode         SP sha1          TAB path
++
+This format is no longer produced by any Git command, but is
+and will continue to be supported by `update-index --index-info`.
+
 To place a higher stage entry to the index, the path should
 first be removed by feeding a mode=0 entry for the path, and
 then feeding necessary input lines in the third format.