Enhance docs for Int#downto (#14176)

Co-authored-by: George Dietrich <george@dietrich.app>

src/int.cr

@@ -567,6 +567,20 @@ struct Int
   end
 
   # Calls the given block with each integer value from self down to `to`.
+  #
+  # ```
+  # 3.downto(1) do |i|
+  #   puts i
+  # end
+  # ```
+  #
+  # Prints:
+  #
+  # ```text
+  # 3
+  # 2
+  # 1
+  # ```
   def downto(to, &block : self ->) : Nil
     return unless self >= to
     x = self