Move to_param undef closer to the deprecations.

Makes it clearer why we're doing this
Add a comment to elaborate on what the undef means and move it closer
to the deprecations so it's less likely for whoever pulls the
deprecations that it should go too.

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -411,8 +411,6 @@ module ActionController
       convert_hashes_to_parameters(key, @parameters[key])
     end
 
-    undef_method :to_param
-
     # Assigns a value to a given +key+. The given key may still get filtered out
     # when +permit+ is called.
     def []=(key, value)
@@ -622,6 +620,10 @@ module ActionController
       end
     end
 
+    # Undefine `to_param` such that it gets caught in the `method_missing`
+    # deprecation cycle below.
+    undef_method :to_param
+
     def method_missing(method_sym, *args, &block)
       if @parameters.respond_to?(method_sym)
         message = <<-DEPRECATE.squish