Documentation/git-update-index: add missing opts to synopsis

Split index related options should appear in the 'SYNOPSIS'
section.

These options are already documented in the 'OPTIONS' section.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-update-index.txt

@@ -17,6 +17,7 @@ SYNOPSIS
 	     [--[no-]assume-unchanged]
 	     [--[no-]skip-worktree]
 	     [--ignore-submodules]
+	     [--[no-]split-index]
 	     [--[no-|force-]untracked-cache]
 	     [--really-refresh] [--unresolve] [--again | -g]
 	     [--info-only] [--index-info]