docs: add docs

Author: kenjis

user_guide_src/source/changelogs/v4.5.0.rst

@@ -268,6 +268,8 @@ Others
   usage in your view files, which was supported by CodeIgniter 3.
 - **CSP:** Added ``ContentSecurityPolicy::clearDirective()`` method to clear
   existing CSP directives. See :ref:`csp-clear-directives`.
+- **HTTP:** Added ``Message::addHeader()`` method to add another header with
+  the same name. See :php:meth:`CodeIgniter\\HTTP\\Message::addHeader()`.
 
 Message Changes
 ***************

user_guide_src/source/incoming/message.rst

@@ -7,7 +7,7 @@ requests and responses, including the message body, protocol version, utilities
 the headers, and methods for handling content negotiation.
 
 This class is the parent class that both the :doc:`Request Class <../incoming/request>` and the
-:doc:`Response Class <../outgoing/response>` extend from.
+:doc:`Response Class <../outgoing/response>` extend from, and it is not used directly.
 
 ***************
 Class Reference
@@ -146,6 +146,20 @@ Class Reference
 
         .. literalinclude:: message/009.php
 
+    .. php:method:: addHeader($name, $value)
+
+        .. versionadded:: 4.5.0
+
+        :param string $name: The name of the header to add.
+        :param string  $value: The value of the header.
+        :returns: The current message instance
+        :rtype: CodeIgniter\\HTTP\\Message
+
+        Adds a header (not a header value) with the same name.
+        Use this only when you set multiple headers with the same name,
+
+        .. literalinclude:: message/011.php
+
     .. php:method:: getProtocolVersion()
 
         :returns: The current HTTP protocol version

user_guide_src/source/incoming/message/011.php

@@ -0,0 +1,4 @@
+<?php
+
+$message->addHeader('Set-Cookie', 'logged_in=no; Path=/');
+$message->addHeader('Set-Cookie', 'sessid=123456; Path=/');