Tuning Guide lacks information on turning off Derby property settings

The documentation of how to turn off Derby properties is spotty. Dag Wanvik reports that "I checked the code, and when you set a database property to a null value, that property is removed from the list of database properties; i.e. the situation is back to where you were before it was first set, so the built-in defaults apply again (or system properties if there are any)."

The behavior for turning off user settings is slightly different and is documented in several places; it's the general case that seems to be missing. This information needs to be added to the description of Derby properties in the Tuning Guide, probably in the general topic "Derby properties" (http://db.apache.org/derby/docs/dev/tuning/ctunproper22250.html).