Update filter documentation

- Add note about filters having become available in 2.1.
- Add insertstream to release notes and change log.
- Add complete example to cache documentation.

Documentation/Filters/Maxrows.md

@@ -1,5 +1,7 @@
 # Maxrows
 
+This filter was introduced in MariaDB MaxScale 2.1.
+
 ## Overview
 The maxrows filter is capable of restricting the amount of rows that a SELECT,
  a prepared statement or stored procedure could return to the client application.