adds some details to the rationale of converted_arrays [ci skip]

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -129,6 +129,10 @@ def initialize(attributes = nil)
     # Attribute that keeps track of converted arrays, if any, to avoid double
     # looping in the common use case permit + mass-assignment. Defined in a
     # method to instantiate it only if needed.
+    #
+    # Testing membership still loops, but it's going to be faster than our own
+    # loop that converts values. Also, we are not going to build a new array
+    # object per fetch.
     def converted_arrays
       @converted_arrays ||= Set.new
     end

actionpack/test/controller/parameters/parameters_permit_test.rb

@@ -176,7 +176,7 @@ def assert_filtered_out(params, key)
   end
 
   # Strong params has an internal cache to avoid duplicated loops in the most
-  # common usage pattern. See the docs of the method `coverted_array`.
+  # common usage pattern. See the docs of the method `converted_arrays`.
   #
   # This test checks that if we push a hash to an array (in-place modification)
   # the cache does not get fooled, the hash is still wrapped as strong params,