doc: document diff/merge.guitool config keys

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -177,6 +177,14 @@ diff.tool::
 	Any other value is treated as a custom diff tool and requires
 	that a corresponding difftool.<tool>.cmd variable is defined.
 
+diff.guitool::
+	Controls which diff tool is used by linkgit:git-difftool[1] when
+	the -g/--gui flag is specified. This variable overrides the value
+	configured in `merge.guitool`. The list below shows the valid
+	built-in values. Any other value is treated as a custom diff tool
+	and requires that a corresponding difftool.<guitool>.cmd variable
+	is defined.
+
 include::mergetools-diff.txt[]
 
 diff.indentHeuristic::

Documentation/merge-config.txt

@@ -63,6 +63,12 @@ merge.tool::
 	Any other value is treated as a custom merge tool and requires
 	that a corresponding mergetool.<tool>.cmd variable is defined.
 
+merge.guitool::
+	Controls which merge tool is used by linkgit:git-mergetool[1] when the
+	-g/--gui flag is specified. The list below shows the valid built-in values.
+	Any other value is treated as a custom merge tool and requires that a
+	corresponding mergetool.<guitool>.cmd variable is defined.
+
 include::mergetools-merge.txt[]
 
 merge.verbosity::