config.txt: fix placement of diff.noprefix

In git-config(1), diff.noprefix was placed in between
diff.mnemonicprefix and the list of mnemonic prefixes, which is
obviously incorrect and very confusing to readers.  Now, it is located
after the end of the explanation of mnemonicprefix, which makes much
more sense.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -804,8 +804,6 @@ diff.mnemonicprefix::
 	standard "a/" and "b/" depending on what is being compared.  When
 	this configuration is in effect, reverse diff output also swaps
 	the order of the prefixes:
-diff.noprefix::
-	If set, 'git diff' does not show any source or destination prefix.
 `git diff`;;
 	compares the (i)ndex and the (w)ork tree;
 `git diff HEAD`;;
@@ -817,6 +815,9 @@ diff.noprefix::
 `git diff --no-index a b`;;
 	compares two non-git things (1) and (2).
 
+diff.noprefix::
+	If set, 'git diff' does not show any source or destination prefix.
+
 diff.renameLimit::
 	The number of files to consider when performing the copy/rename
 	detection; equivalent to the 'git diff' option '-l'.