Backing up read-only dbspaces [CR #631508]

The following was added to SQL Statements > BACKUP Statement in the paragraph preceding CRC clause:

Once you back up a read-only dbspace that you do not plan to change, you may omit it from subsequent backups, reduce time to back up, and save space. Be careful; changing such a dbspace back to read-write at any time after the backup invalidates the last read-only backup.