doc: align 'diff --no-index' in text and synopsis

Make the two '<path>' parameters in DESCRIPTION mandatory and
move the `--options` part to the same place where the other
variants show them. And finally make `--no-index` in SYNOPSIS
as mandatory as in DESCRIPTION.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Reviewed-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -13,7 +13,7 @@ SYNOPSIS
 'git diff' [options] --cached [<commit>] [--] [<path>...]
 'git diff' [options] <commit> <commit> [--] [<path>...]
 'git diff' [options] <blob> <blob>
-'git diff' [options] [--no-index] [--] <path> <path>
+'git diff' [options] --no-index [--] <path> <path>
 
 DESCRIPTION
 -----------
@@ -29,7 +29,7 @@ two blob objects, or changes between two files on disk.
 	further add to the index but you still haven't.  You can
 	stage these changes by using linkgit:git-add[1].
 
-'git diff' --no-index [--options] [--] [<path>...]::
+'git diff' [--options] --no-index [--] <path> <path>::
 
 	This form is to compare the given two paths on the
 	filesystem.  You can omit the `--no-index` option when