docs: add set-cookie special case (#3470)

* docs: add set-cookie special case

* add sub section about set-cookie

* fix typos and add point about resetting set-cookie

docs/Reply.md

@@ -7,6 +7,7 @@
   - [.statusCode](#statusCode)
   - [.server](#server)
   - [.header(key, value)](#headerkey-value)
+      - [set-cookie](#set-cookie)
   - [.headers(object)](#headersobject)
   - [.getHeader(key)](#getheaderkey)
   - [.getHeaders()](#getheaders)
@@ -109,6 +110,18 @@ fastify.get('/', async function (req, rep) {
 Sets a response header. If the value is omitted or undefined, it is coerced
 to `''`.
 
+<a name="set-cookie"></a>
+- ### set-cookie
+    - While sending different values as cookie with `set-cookie` as the key, every value will be sent as cookie instead of replacing the previous value.
+
+    ```js
+    reply.header('set-cookie', 'foo');
+    reply.header('set-cookie', 'bar');
+    ```
+  - The browser will only consider the latest reference of a key for `set-cookie` header. The fact that this is done this way is to avoid parsing the set-cookie header when you add it in the reply and speeds up the serialization of the reply.
+
+  - To reset the `set-cookie`, you need to make an explicit call to `reply.removeHeader('set-cookie')`, read more about `.removeHeader(key)` [here](#removeheaderkey).
+
 For more information, see [`http.ServerResponse#setHeader`](https://nodejs.org/dist/latest-v14.x/docs/api/http.html#http_response_setheader_name_value).
 
 <a name="headers"></a>