Improve docs for interaction.Select

Attempts to make the options clearer.

I find the current option names 'condition' and 'toggle' rather confusing. ISTM that 'condition' should be called 'toggle' (a simple select/unselect toggle), and 'toggle' should be called 'modifier' (or something similar).
I do not understand "By default, {@link ol.events.condition.singleClick} makes this feature the selected one." Is it just me?