diff --git a/Library/Homebrew/formula_pin.rb b/Library/Homebrew/formula_pin.rb
index f9c54097fa..613a51fc81 100644
--- a/Library/Homebrew/formula_pin.rb
+++ b/Library/Homebrew/formula_pin.rb
@@ -2,6 +2,9 @@
 
 require "keg"
 
+# Helper functions for pinning a formula.
+#
+# @api private
 class FormulaPin
   def initialize(f)
     @f = f