docs: response.set(fields) won't overwrites previous header fields(#1282)

Author: doug-wade

docs/api/response.md

@@ -193,6 +193,8 @@ ctx.set({
 });
 ```
 
+This delegates to [setHeader](https://nodejs.org/dist/latest/docs/api/http.html#http_request_setheader_name_value) which sets or updates headers by specified keys and doesn't reset the entire header.
+
 ### response.remove(field)
 
   Remove header `field`.