utils: Add a comment to module Kernel to appease RuboCop

- This `Homebrew/utils.rb` file contains one `@api public` method so it's now
  included in `Style/Documentation`.
- This method not having a comment was causing the style specs to fail because
  this file isn't usually failing RuboCop.
- And the test description was confusing so I improved it.

Library/Homebrew/extend/kernel.rb

@@ -1,6 +1,7 @@
 # typed: false
 # frozen_string_literal: true
 
+# Contains shorthand Homebrew utility methods like `ohai`, `opoo`, `odisabled`.
 module Kernel
   extend T::Sig
 

Library/Homebrew/test/style_spec.rb

@@ -40,7 +40,7 @@ describe Homebrew::Style do
   describe ".check_style_and_print" do
     let(:dir) { mktmpdir }
 
-    it "returns false for conforming file with only audit-level violations" do
+    it "returns true (success) for conforming file with only audit-level violations" do
       # This file is known to use non-rocket hashes and other things that trigger audit,
       # but not regular, cop violations
       target_file = HOMEBREW_LIBRARY_PATH/"utils.rb"