format_completion: Move 'as' and 'esc_mode' to options (second argument) to avoid cluttering the completion answer structure. 'esc_mode' is no longer in the specification, to avoid abstraction leak.
Switch the meaning of the escape modes 'default' and 'shellvar', so now in 'default' the dollar sign is not escaped while in 'shellvar' it is. This is more convenient in general.
Documentation
Remove explanation for 'path_sep' (moved to the Complete specification).