Add comment about implementation of class_attribute

To prevent future pull requests like #8435 add a comment about the
implementation of class_attribute using class_eval for performance.

[ci skip]

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -72,6 +72,9 @@ class Class
     instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
     instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
 
+    # We use class_eval here rather than define_method because class_attribute
+    # may be used in a performance sensitive context therefore the overhead that
+    # define_method introduces may become significant.
     attrs.each do |name|
       class_eval <<-RUBY, __FILE__, __LINE__ + 1
         def self.#{name}() nil end