difftool docs: restore correct position of tool list

2a9dfdf (difftool docs: de-duplicate configuration sections, 2022-09-07)
moved the difftool documentation, but missed moving this "include" line that
includes the generated list of diff tools, as referenced in the moved text.
This patch restores the correct position of the included list.

Signed-off-by: Adam Johnson <me@adamj.eu>

Documentation/config/diff.txt

@@ -218,8 +218,6 @@ endif::git-diff[]
 	Set this option to `true` to make the diff driver cache the text
 	conversion outputs.  See linkgit:gitattributes[5] for details.
 
-include::{build_dir}/mergetools-diff.txt[]
-
 `diff.indentHeuristic`::
 	Set this option to `false` to disable the default heuristics
 	that shift diff hunk boundaries to make patches easier to read.

Documentation/config/difftool.txt

@@ -13,6 +13,8 @@ diff.guitool::
 	and requires that a corresponding difftool.<guitool>.cmd variable
 	is defined.
 
+include::{build_dir}/mergetools-diff.txt[]
+
 difftool.<tool>.cmd::
 	Specify the command to invoke the specified diff tool.
 	The specified command is evaluated in shell with the following