[ci skip] Clarify why we're clearing reporters.

Minitest by default includes a summary reporter and a progress reporter.

To print colored output, we have to replace the progress reporter.

railties/lib/rails/test_unit/minitest_plugin.rb

@@ -86,7 +86,7 @@ def self.plugin_rails_init(options)
     # Disable the extra failure output after a run, unless output is deferred.
     self.hide_aggregated_results = options[:output_inline]
 
-    self.reporter.reporters.clear
+    self.reporter.reporters.clear # Replace progress reporter for colors.
     self.reporter << SummaryReporter.new(options[:io], options)
     self.reporter << ::Rails::TestUnitReporter.new(options[:io], options)
   end