Explicitly mark non-public APIs.

Library/Homebrew/exceptions.rb

@@ -49,6 +49,9 @@ class UnsupportedInstallationMethod < RuntimeError; end
 
 class MultipleVersionsInstalledError < RuntimeError; end
 
+# Raised when a path is not a keg.
+#
+# @api internal
 class NotAKegError < RuntimeError; end
 
 # Raised when a keg doesn't exist.

Library/Homebrew/extend/kernel.rb

@@ -255,7 +255,9 @@ module Kernel
     raise ErrorDuringExecution.new([cmd, *args], status: $CHILD_STATUS)
   end
 
-  # Prints no output.
+  # Run a system comand without any output.
+  #
+  # @api internal
   def quiet_system(cmd, *args)
     Homebrew._system(cmd, *args) do
       # Redirect output streams to `/dev/null` instead of closing as some programs

Library/Homebrew/system_command.rb

@@ -12,14 +12,22 @@ require "utils/timer"
 
 # Class for running sub-processes and capturing their output and exit status.
 #
-# @api private
+# @api internal
 class SystemCommand
   # Helper functions for calling {SystemCommand.run}.
+  #
+  # @api internal
   module Mixin
+    # Run a fallible system command.
+    #
+    # @api internal
     def system_command(executable, **options)
       SystemCommand.run(executable, **options)
     end
 
+    # Run an infallible system command.
+    #
+    # @api internal
     def system_command!(command, **options)
       SystemCommand.run!(command, **options)
     end

Library/Homebrew/utils/formatter.rb

@@ -14,6 +14,9 @@ module Formatter
     prefix("==>", string, color)
   end
 
+  # Format a string as headline.
+  #
+  # @api internal
   def self.headline(string, color: nil)
     arrow("#{Tty.bold}#{string}#{Tty.reset}", color:)
   end