ref-filter: add docs for new options

diff --git a/Documentation/diff-config.txt b/Documentation/diff-config.txt
index 85bca83c304c6d95422482b9bc7b8d983bfdc85e..e64d983c344656dd00d869005563a1a20efce854 100644 (file)
--- a/Documentation/diff-config.txt
+++ b/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::