improve documentation of `FilteredDataset$get_call`

[chlebowa]

This saegment is very unclear, please rewrite.

    #' @description
    #' Gets a filter expression.
    #'
    #' This function returns filter calls equivalent to selected items
    #' within each of `filter_states`. Configuration of the calls is constant and
    #' depends on `filter_states` type and order which are set during initialization.
    #'
    #' @param sid (`character`)
    #'  when specified, the method returns code containing conditions calls of
    #'  `FilterState` objects with `sid` different to this `sid` argument.