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).
only if used in combination with the condition key (click by default). Just pressing the shift key on its own doesn't change anything.