Documentation: git-gui: describe gui.displayuntracked

Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1340,6 +1340,10 @@ gui.diffcontext::
 	Specifies how many context lines should be used in calls to diff
 	made by the linkgit:git-gui[1]. The default is "5".
 
+gui.displayuntracked::
+	Determines if linkgit::git-gui[1] shows untracked files
+	in the file list. The default is "true".
+
 gui.encoding::
 	Specifies the default encoding to use for displaying of
 	file contents in linkgit:git-gui[1] and linkgit:gitk[1].