Better session documentation and link (#885)

* Better session documentation and link

Update seasoning documentation hint and link the chapter on sessions

* - clean up and expand

Co-authored-by: Jens Vagelpohl <jens@netz.ooo>

docs/migrations/zope4/removed.rst

@@ -24,19 +24,35 @@ its ``eggs`` attribute and also add the flag ``wsgi = off``.
 Sessioning
 ----------
 If you have used (or want to use) the built-in support for sessioning, add the
-egg ``Products.Sessions`` to your application buildout.
+egg ``Products.Sessions`` to your application buildout, which provides the
+basic infrastructure without the actual session data storage.
 
-You also need to make sure that your Zope configuration file contains a ZODB
+For non-production environments you can use the old
+``Products.TemporaryFolder`` temporary folder solution for storing session
+data. But this implementation is known to randomly lose session data, so do not
+use it in production. Add the package ``Products.TemporaryFolder`` to your
+application buildout and make sure your Zope configuration file contains a ZODB
 configuration for a temporary folder like this::
 
   <zodb_db temporary>
-      <temporarystorage>
+      <mappingstorage>
         name Temporary database (for sessions)
-      </temporarystorage>
+      </mappingstorage>
       mount-point /temp_folder
       container-class Products.TemporaryFolder.TemporaryContainer
   </zodb_db>
 
+If sessions are used very sparingly you can even get away with just adding a
+Folder object named ``temp_folder`` at the root of the ZODB and restarting
+Zope so the necessary ZODB objects for session support are created. This will
+not lose session data, but it has a high risk of producing ZODB conflict errors
+when storing data unless the session is used very carefully to minimize write
+activity.
+
+For production deployments see, see `the Zope book chapter on sessioning
+for alternative session storage options
+<https://zope.readthedocs.io/en/latest/zopebook/Sessions.html#alternative-server-side-session-backends-for-zope-4>`_.
+
 
 External Methods
 ----------------