Documented cookie problem for #80

docs/config.rst

@@ -64,6 +64,25 @@ The following configuration values are used internally by Flask:
                                 subdomain support (eg: ``'localhost'``)
 =============================== =========================================
 
+.. admonition:: More on ``SERVER_NAME``
+
+   The ``SERVER_NAME`` key is used for the subdomain support.  Because
+   Flask cannot guess the subdomain part without the knowledge of the
+   actual server name, this is required if you want to work with
+   subdomains.  This is also used for the session cookie.
+
+   Please keep in mind that not only Flask has the problem of not knowing
+   what subdomains are, your web browser does as well.  Most modern web
+   browsers will not allow cross-subdomain cookies to be set on a
+   server name without dots in it.  So if your server name is
+   ``'localhost'`` you will not be able to set a cookie for
+   ``'localhost'`` and every subdomain of it.  Please chose a different
+   server name in that case, like ``'myapplication.local'`` and add
+   this name + the subdomains you want to use into your host config
+   or setup a local `bind`_.
+
+.. _bind: https://www.isc.org/software/bind
+
 .. versionadded:: 0.4
    ``LOGGER_NAME``