docs: add a note for confusing behavior

Author: kenjis

user_guide_src/source/helpers/cookie_helper.rst

@@ -69,7 +69,13 @@ The following functions are available:
     .. literalinclude:: cookie_helper/002.php
 
     This function is otherwise identical to ``set_cookie()``, except that it
-    does not have the ``value`` and ``expire`` parameters. You can submit an
+    does not have the ``value`` and ``expire`` parameters.
+
+    .. note:: When you use ``set_cookie()``,
+        if the ``value`` is set to empty string and the ``expire`` is set to ``0``, the cookie will be deleted.
+        If the ``value`` is set to non-empty string and the ``expire`` is set to ``0``, the cookie will only last as long as the browser is open.
+
+    You can submit an
     array of values in the first parameter or you can set discrete
     parameters.
 

user_guide_src/source/outgoing/response.rst

@@ -394,6 +394,9 @@ The methods provided by the parent class that are available are:
         from *now* that you wish the cookie to be valid. If the ``expire`` is
         set to zero the cookie will only last as long as the browser is open.
 
+        .. note:: But if the ``value`` is set to empty string and the ``expire`` is set to ``0``,
+            the cookie will be deleted.
+
         For site-wide cookies regardless of how your site is requested, add your
         URL to the ``domain`` starting with a period, like this:
         .your-domain.com