Add DB closed recommendation to docs for Database.copy

As long as a database is open, it is possible that one or more of its files will be in use (this is especially on the Windows platform).  This will cause the copy to fail with a : "Device or resource busy"

The inline documentation for the Database.copy method should recommend that the source database be closed before a call.

The Web documentation uses the method in the single case of copying a pre-packaged db.  Pre-packaged DBs are probably never open, so the web doc does not need updating.