Prefs questions

The DT 1.9.1 manual (p.42) says:

Another possibility is to tell DEVONthink to save modified documents at specified intervals, see “Preferences… > Database > Auto flush to disk every…”.

I see no such pref in the prefs panel.

Also, I see no pref for automatic smart quotes while typing into plain text and RTF files. Is there one?

This options has been removed as DEVONthink 1.9.1 and later automatically flush to disk every three minutes. And no, there is no such preference option for smart quotes at the moment.