Clarify documentation for system test arguments

Previously this implied that system tests provided other non-headless
drivers when Selenium is the only driver that uses `:using`,
`:screen_size` or `:options` arguments.

This change clarifies that Selenium is the only non-headless driver.

guides/source/testing.md

@@ -658,8 +658,8 @@ end
 
 The driver name is a required argument for `driven_by`. The optional arguments
 that can be passed to `driven_by` are `:using` for the browser (this will only
-be used for non-headless drivers like Selenium), and `:screen_size` to change
-the size of the screen for screenshots.
+be used by Selenium), and `:screen_size` to change the size of the screen for
+screenshots.
 
 ```ruby
 require "test_helper"