Document the --no-gui option in difftool

Prior to this, the `--no-gui` option was not documented in the manpage.
This commit introduces this into the manpage

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

Documentation/git-difftool.txt

@@ -86,10 +86,11 @@ instead.  `--no-symlinks` is the default on Windows.
 	Additionally, `$BASE` is set in the environment.
 
 -g::
---gui::
+--[no-]gui::
 	When 'git-difftool' is invoked with the `-g` or `--gui` option
 	the default diff tool will be read from the configured
-	`diff.guitool` variable instead of `diff.tool`.
+	`diff.guitool` variable instead of `diff.tool`. The `--no-gui`
+	option can be used to override this setting.
 
 --[no-]trust-exit-code::
 	'git-difftool' invokes a diff tool individually on each file.