Provide better documentation for IPreferencePanelProvider

[josh@…]

Having just spent the last half-day hacking around trying to get a plugin to work, I believe the the IPreferencePanelProvider interface either needs a function specifically for setting up data to be saved into the DB, or some better documentation informing developers that they need to add something like this to render_preference_panel in order to get things to save into the DB.

session = req.session
if req.method == "POST":
    session['var'] = req.args.get('var', '')
data = dict(homepage = session.get('var', ''),)
return 'prefs_template.html', data