Documentation: Move "git diff <blob> <blob>"

The section describing "git diff <blob> <blob>" had been placed in a
position that disrupted the statement "This is synonymous to the
previous form".

Reorder to place this form after all the <commit>-using forms, and the
note applying to them. Also mention this form in the initial description
paragraph.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff.txt

@@ -18,8 +18,8 @@ SYNOPSIS
 DESCRIPTION
 -----------
 Show changes between the working tree and the index or a tree, changes
-between the index and a tree, changes between two trees, or changes
-between two files on disk.
+between the index and a tree, changes between two trees, changes between
+two blob objects, or changes between two files on disk.
 
 'git diff' [--options] [--] [<path>...]::
 
@@ -56,11 +56,6 @@ directories. This behavior can be forced by --no-index.
 	This is to view the changes between two arbitrary
 	<commit>.
 
-'git diff' [options] <blob> <blob>::
-
-	This form is to view the differences between the raw
-	contents of two blob objects.
-
 'git diff' [--options] <commit>..<commit> [--] [<path>...]::
 
 	This is synonymous to the previous form.  If <commit> on
@@ -87,6 +82,11 @@ and the range notations ("<commit>..<commit>" and
 "<commit>\...<commit>") do not mean a range as defined in the
 "SPECIFYING RANGES" section in linkgit:gitrevisions[7].
 
+'git diff' [options] <blob> <blob>::
+
+	This form is to view the differences between the raw
+	contents of two blob objects.
+
 OPTIONS
 -------
 :git-diff: 1