Add a comment about Integer#downto Enumerator

Merged: https://github.com/ruby/ruby/pull/12074

numeric.rb

@@ -329,6 +329,8 @@ class Integer
       def downto(to) # :nodoc:
         Primitive.attr! :inline_block, :c_trace
 
+        # When no block is given, return an Enumerator that enumerates from `self` to `to`.
+        # Not using `block_defined?` and `to_enum` to keep them unaffected by redefinitions.
         unless defined?(yield)
           return Primitive.cexpr! 'SIZED_ENUMERATOR(self, 1, &to, int_downto_size)'
         end