Update the metadata_update_threshold description

Author: javiereguiluz

reference/configuration/framework.rst

@@ -1024,12 +1024,9 @@ setting the value to ``null``:
 
 **type**: ``integer`` **default**: ``0``
 
-This is how many seconds to wait between updating/writing the session metadata. This
-can be useful if, for some reason, you want to limit the frequency at which the
-session persists.
-
-Starting in Symfony 3.4, session data is *only* written when the session data has
-changed. Previously, you needed to set this option to avoid that behavior.
+This is how many seconds to wait between updating/writing the session metadata.
+This can be useful if, for some reason, you want to limit the frequency at which
+the session persists, instead of doing that on every request.
 
 .. _reference-session-enabled: