Document OS.

Library/Homebrew/os.rb

@@ -1,18 +1,30 @@
 # frozen_string_literal: true
 
+# Helper functions for querying operating system information.
+#
+# @api private
 module OS
+  # Check if the operating system is macOS.
+  #
+  # @api public
   def self.mac?
     return false if ENV["HOMEBREW_TEST_GENERIC_OS"]
 
     RbConfig::CONFIG["host_os"].include? "darwin"
   end
 
+  # Check if the operating system is Linux.
+  #
+  # @api public
   def self.linux?
     return false if ENV["HOMEBREW_TEST_GENERIC_OS"]
 
     RbConfig::CONFIG["host_os"].include? "linux"
   end
 
+  # Get the kernel version.
+  #
+  # @api public
   def self.kernel_version
     @kernel_version ||= Version.new(Utils.safe_popen_read("uname", "-r").chomp)
   end