ls-tree manpage: output of ls-tree is compatible with update-index

Such format relationships are very useful things to remember for
script writers.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-tree.txt

@@ -78,6 +78,8 @@ Output Format
 
 Unless the `-z` option is used, TAB, LF, and backslash characters
 in pathnames are represented as `\t`, `\n`, and `\\`, respectively.
+This output format is compatible with what '--index-info --stdin' of
+'git update-index' expects.
 
 When the `-l` option is used, format changes to